Require authors to set featured image for post

You need to hook the Publish Action in a custom PlugIn you write. Although this is to require a title, this should get you started, you just need to check if a featured image was assigned.

add_action( 'pre_post_update', 'bawdp_dont_publish' );function bawdp_dont_publish(){    global $post;    if ( strlen( $post->title ) < 10 ) {        wp_die( 'The title of your post have to be 10 or more !' );    }}

Look at (has_post_thumbnail( $post->ID )) to determine if a post has a featured image.

Given Gary's example above, I've written the following to my functions.php file:

function featured_image_requirement() {     if(!has_post_thumbnail()) {          wp_die( 'You forgot to set the featured image. Click the back button on your browser and set it.' );      } }add_action( 'pre_post_update', 'featured_image_requirement' );

I'd much rather see this in a plugin as well - there's one called Mandatory Field but it doesn't work with scheduled posts. Both are not really eloquent solutions.

you can use a plugin

https://wordpress.org/plugins/require-featured-image/

or you can copy and paste below code in your wordpress theme functions.php file:

<?php/** * Require a featured image to be set before a post can be published. */add_filter( 'wp_insert_post_data', function ( $data, $postarr ) {    $post_id              = $postarr['ID'];    $post_status          = $data['post_status'];    $original_post_status = $postarr['original_post_status'];    if ( $post_id && 'publish' === $post_status && 'publish' !== $original_post_status ) {        $post_type = get_post_type( $post_id );        if ( post_type_supports( $post_type, 'thumbnail' ) && ! has_post_thumbnail( $post_id ) ) {            $data['post_status'] = 'draft';        }    }    return $data;}, 10, 2 );add_action( 'admin_notices', function () {    $post = get_post();    if ( 'publish' !== get_post_status( $post->ID ) && ! has_post_thumbnail( $post->ID ) ) { ?>        <div id="message" class="error">            <p>                <strong><?php _e( 'Please set a Featured Image. This post cannot be published without one.' ); ?></strong>            </p>        </div>    <?php    }} );