Warning: The following meta box are not available: "meta-box-id".

Hi,

That means there is no meta box that has the ID meta-box-id. It's the sample value for the ID of the registration shortcode. You need to replace it with your activate field group ID https://docs.metabox.io/extensions/meta-box-builder/#creating-a-field-group

The attribute ID of the registration form is optional, you can remove it as you want.

Hi,

The new version of MB User Profile shows the warning when the meta box ID is not available but you are using it in the shortcode. Please just remove it.

Docs:

[mb_user_profile_register id="meta-box-id" label_submit="Register" confirmation="Your account has been created successfully."]

change to:

[mb_user_profile_register label_submit="Register" confirmation="Your account has been created successfully."]