How to show custom post type by either single meta_value or multiple meta_value(s) using WP_Query?

Since $course_only_ids is already an array, you shouldn’t be nesting it inside another:

array( $course_only_ids );
// array( array( 2031, 300, 1919 ) );

Just use:

'value' => $course_only_ids,

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)