add_filter( 'streamtube/core/post/update/errors', function( $errors ){

    if( ! array_key_exists( 'categories', $_POST['tax_input'] ) ){
        $errors->add(
            'no_category',
            esc_html__( 'Category is required', 'streamtube-child' )
        );
    }

    return $errors;

}, 10, 1 );