Hi Yumikom,
Maybe the theme or the plugin you're using has a call to the function "rwmb_meta()" when the Meta Box plugin is not loaded (or activated). It's better to change the code to:
if ( function_exists( 'rwmb_meta' ) ) echo rwmb_meta( 'field_id' );
If your field is a simple field (like text or textarea), you should use get_post_meta(), like this:
echo get_post_meta( get_the_ID(), 'field_id', true );
rwmb_meta() is just a wrapper of get_post_meta. Using get_post_meta might give you a better performance.
.
