Require Post Category

Nope, just install and activate, that’s it!

Use the rpc_post_types filter hook in your theme’s functions.php or a must-use plugin.

Usage examples:

function custom_rpc_post_types( $post_types ) {
    // Add a key to the $post_types array for each post type and list the slugs of the taxonomies you wish to require

    // Simplest usage
    $post_types['book'] = array( 'genre' );

    // Multiple taxonomies
    $post_types['recipe'] = array( 'cookbook_category', 'geographic_origin', 'flavor_tags' );

    // Set your own alert message for each taxonomy, or let the plugin generate the alert message
    $post_types['inventory'] = array(
        // Let the plugin generate a relevant alert message
        'manufacturer',
        // Or specify a custom alert message
        'inventory_category' => array(
            'message' => "Please choose a category for this fine inventory item."
        )
    );

    // Always return $post_types after your modifications
    return $post_types;
}

add_filter( 'rpc_post_types', 'custom_rpc_post_types' );

The default $post_types contains the following:

$post_types['post'] = array(
    'category' => array(
        'message' => 'Please select a category before publishing this post.'
    )
);

This maintains the plugin’s original functionality. However, you can remove this functionality with unset($post_types['post']); or by redefining $post_types in your hook function.

One reason may be that your custom taxonomy does not have the show_in_rest argument enabled. Without it enabled the Gutenberg editor cannot access it.

Visit this plugin’s WordPress support forum at https://wordpress.org/support/plugin/require-post-category