The Video Files/URLs should always be added using their own fields available in the video forms. For this reason, the “Add Media” button is disabled on Video forms to prevent users from adding videos in the description box. But, you can enable it back by adding the following code to the bottom of your theme’s functions.php file:
// Add “Add Media” button on the Edit Video post
function aiovg_add_media_buttons() {
global $current_screen;
if ( 'aiovg_videos' == $current_screen->post_type ) add_action( 'media_buttons', 'media_buttons' );
}
add_action( 'admin_head', 'aiovg_add_media_buttons', 99 );