Name to undefined perform wp_create_category()

I’m sorry, I converse a bit English.

My customized theme:

~/wp-content/themes/mycustomtheme/capabilities.php

perform mct_task() {
    embrace( get_template_directory() . '/inc/cron.php' );
}
add_action( 'mct_hook', 'mct_task' );
//
perform mytheme_setup_options() {
    if( ! wp_next_scheduled( 'mct_hook' ) ) {
        wp_schedule_event( time(), 'hourly', 'mct_hook' );
    } else {}
}
add_action( 'after_switch_theme', 'mytheme_setup_options' );

~/wp-content/themes/mycustomtheme/inc/cron.php

$category_id = wp_create_category( 'Tremendous' );
// $my_post = array( 'post_category'=> array( $category_id ) );
wp_insert_post( $my_post );

~/error_log

PHP Deadly error: Uncaught Error: Name to undefined perform wp_create_category() in...

Why? What’s the downside?

I would love use class at wp_insert_post with wp-cron. If the class exists, use the class ID, if not exists, create new class.