Use Image Upload field to set the Featured Image of the post

Hi Long - can you break this down a bit for me?

I have a custom field set for a CPT. This set includes an Image Upload with these settings:
- ID: _thumbnail_id
- Max number of files: 1

I have a snippet with the following where "candidate" is the Field Group ID:

add_filter( 'rwmb_frontend_validate', function( $validate, $config ) {
    if ( 'candidate' !== $config['_thumbnail_id'] ) {
        return $validate;
    }
    if ( empty( $_POST['image_upload_field'] ) ) {
        $validate = 'Please select at least one image to upload'; // Return a custom error message
    }
    return $validate;
}, 10, 2 );

Can you tell me where I went wrong? The image is not saving to the post at all (as a regular field or as a Featured Image). This is true when using the front-end form or when editing the post from the backend.

Thanks in advance for your help!

Thanks Long -

I made the changes but the image is still not saving to the post, or as a featured image.
It is, however, saving to the Media Library.

add_filter( 'rwmb_frontend_validate', function( $validate, $config ) {
    if ( 'candidate' !== $config['id'] ) {
        return $validate;
    }
    if ( empty( $_POST['_thumbnail_id'] ) ) {
        $validate = 'Please select at least one image to upload'; // Return a custom error message
    }
    return $validate;
}, 10, 2 );

In case it's useful, here are screenshots of the custom field:
https://www.awesomescreenshot.com/image/28499524?key=8ff840b5f2d8f12f5597455d36426661
https://www.awesomescreenshot.com/image/28499595?key=21ed4f1eb276c229c449d473d04f1a16

Hi Long - see below. I did notice that I had "candidate" instead of "candidates" in my snippet. I updated that but it didn't fix the issue.

The CPT is "Candidate" and featured image is enabled.
The Custom field group id is "Candidate"

For the field group:

<?php
add_filter( 'rwmb_meta_boxes', 'your_prefix_function_name' );

function your_prefix_function_name( $meta_boxes ) {
    $prefix = '';

    $meta_boxes[] = [
        'title'      => __( 'Candidates', 'your-text-domain' ),
        'id'         => 'candidates',
        'post_types' => ['candidate'],
        'fields'     => [
            [
                'name' => __( 'First Name', 'your-text-domain' ),
                'id'   => $prefix . 'first_name',
                'type' => 'text',
            ],
            [
                'name' => __( 'Last Name', 'your-text-domain' ),
                'id'   => $prefix . 'last_name',
                'type' => 'text',
            ],
            [
                'name'             => __( 'Headshot', 'your-text-domain' ),
                'id'               => $prefix . ' _thumbnail_id',
                'type'             => 'image_upload',
                'image_size'       => 'medium_large',
                'max_file_uploads' => 1,
                'force_delete'     => true,
                'max_status'       => false,
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'              => __( 'Candidate Statement', 'your-text-domain' ),
                'id'                => $prefix . 'candidate_statement',
                'type'              => 'wysiwyg',
                'label_description' => __( 'Please tell us why you wish to service in this office. You may include actions you wish to promote, or anything you would like members to know. Limit 500 words.', 'your-text-domain' ),
                'required'          => true,
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'              => __( 'Relevant Experience', 'your-text-domain' ),
                'id'                => $prefix . 'relevant_experience',
                'type'              => 'wysiwyg',
                'label_description' => __( 'Please describe your relevant experience - what you bring to the office. Limit 500 words.', 'your-text-domain' ),
                'required'          => true,
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'   => __( 'Social Media', 'your-text-domain' ),
                'id'     => $prefix . 'social_media',
                'type'   => 'group',
                'fields' => [
                    [
                        'name' => __( 'LinkedIn Profile', 'your-text-domain' ),
                        'id'   => $prefix . 'linkedin_profile',
                        'type' => 'url',
                    ],
                    [
                        'name' => __( 'Twitter Handle', 'your-text-domain' ),
                        'id'   => $prefix . 'twitter_handle',
                        'type' => 'url',
                    ],
                    [
                        'name' => __( 'Facebook Profile', 'your-text-domain' ),
                        'id'   => $prefix . 'facebook_profile',
                        'type' => 'url',
                    ],
                    [
                        'name' => __( 'Reddit Handle', 'your-text-domain' ),
                        'id'   => $prefix . 'reddit_handle',
                        'type' => 'url',
                    ],
                ],
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'       => __( 'Organizational Affiliations', 'your-text-domain' ),
                'id'         => $prefix . 'organizational_affiliations',
                'type'       => 'group',
                'clone'      => true,
                'add_button' => __( 'Add another organization', 'your-text-domain' ),
                'fields'     => [
                    [
                        'name' => __( 'Name of Organization', 'your-text-domain' ),
                        'id'   => $prefix . 'name_of_organization',
                        'type' => 'text',
                    ],
                    [
                        'name' => __( 'Your Role', 'your-text-domain' ),
                        'id'   => $prefix . 'your_role',
                        'type' => 'text',
                    ],
                ],
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'     => __( 'Skills and experience you can bring', 'your-text-domain' ),
                'id'       => $prefix . 'skills_and_experience_you_can_bring',
                'type'     => 'checkbox_list',
                'options'  => [
                    'Board development'               => __( 'Board development', 'your-text-domain' ),
                    'Program development'             => __( 'Program development', 'your-text-domain' ),
                    'Financial management'            => __( 'Financial management', 'your-text-domain' ),
                    'Staffing / HR'                   => __( 'Staffing / HR', 'your-text-domain' ),
                    'Fundraising'                     => __( 'Fundraising', 'your-text-domain' ),
                    'Strategic planning'              => __( 'Strategic planning', 'your-text-domain' ),
                    'Lobbying/Legislation'            => __( 'Lobbying/Legislation', 'your-text-domain' ),
                    'Training'                        => __( 'Training', 'your-text-domain' ),
                    'Marketing'                       => __( 'Marketing', 'your-text-domain' ),
                    'Volunteer management'            => __( 'Volunteer management', 'your-text-domain' ),
                    'Other Skills (not listed above)' => __( 'Other Skills (not listed above)', 'your-text-domain' ),
                ],
                'required' => true,
            ],
            [
                'name'              => __( 'Additional skill not listed above', 'your-text-domain' ),
                'id'                => $prefix . 'additional_skill_not_listed_above',
                'type'              => 'text',
                'label_description' => __( 'One per line', 'your-text-domain' ),
                'clone'             => true,
                'add_button'        => __( 'Add another skill', 'your-text-domain' ),
            ],
            [
                'type' => 'divider',
            ],
            [
                'name'     => __( 'Declaration of Accuracy', 'your-text-domain' ),
                'id'       => $prefix . 'declaration_of_accuracy',
                'type'     => 'checkbox_list',
                'options'  => [
                    'I hereby declare that the information provided is true and correct. I also understand that any willful dishonesty may lead to removal from the election process and the NSITSP at large.' => __( 'I hereby declare that the information provided is true and correct. I also understand that any willful dishonesty may lead to removal from the election process and the NSITSP at large.', 'your-text-domain' ),
                ],
                'required' => true,
            ],
        ],
    ];

    return $meta_boxes;
}

My current validation snippet:

add_filter( 'rwmb_frontend_validate', function( $validate, $config ) {
    if ( 'candidates' !== $config['id'] ) {
        return $validate;
    }
    if ( empty( $_POST['_thumbnail_id'] ) ) {
        $validate = 'Please select at least one image to upload'; // Return a custom error message
    }
    return $validate;
}, 10, 2 );