Calling a WordPress Custom JavaScript file

You’re using the function wp_enqueue_scripts() where you should be using wp_enqueue_script().

function my_custom_javascript(){ // Creates the function
    wp_enqueue_script(
        'my_custom_javascript',
        plugin_dir_url(__FILE__) . '/assets/js/custom.js',
        array('jquery')
    ); // Loads the script into the function
}
add_action('wp_enqueue_scripts', 'my_custom_javascript'); 
// calls the function where the script is located

wp_enqueue_script() enqueues the script; wp_enqueue_scripts() is a wrapper for do_action( 'wp_enqueue_scripts' ).