Force Featured Image

somewhere-in-functions.php

<?php

add_action( 'save_post', 'force_check_thumbnail') ;
add_action( 'admin_notices', 'show_thumbnail_error' );
/**
 * Force Featured Image
 * 
 * @link http://wpdevsnippets.com/require-post-thumbnail-uploaded-before-publishing/
 * @param  global $post_id 
 * @return 
 */
function force_check_thumbnail( $post_id ) {

    // change to any custom post type 
    if (get_post_type( $post_id ) != 'post' )
        return;
    
    if ( ! has_post_thumbnail( $post_id ) ) {
        // set a transient to show the users an admin message
        set_transient( "has_post_thumbnail", "no" );
        // unhook this function so it doesn't loop infinitely
        remove_action( 'save_post', 'wpds_check_thumbnail' );
        // update the post set it to draft
        wp_update_post ( array( 'ID' => $post_id, 'post_status' => 'draft' ) );

        add_action( 'save_post', 'force_check_thumbnail' );
    } else {
        delete_transient( "has_post_thumbnail" );
    }
}

function show_thumbnail_error() {
    // check if the transient is set, and display the error message
    if ( get_transient( "has_post_thumbnail" ) == "no" ) {
        echo "<div id='message' class='error'><p><strong>You must select Post Thumbnail. Your Post is saved but it can not be published.</strong></p></div>";
        delete_transient( "has_post_thumbnail" );
    }

}