https://developer.wordpress.org/reference/functions/plugin_dir_path/
It looks like this function adds a trailing slash, so starting your string with a slash is not required try;
require_once(plugin_dir_path( __FILE__ ) . 'assets/MCAPI.class.php');
https://developer.wordpress.org/reference/functions/plugin_dir_path/
It looks like this function adds a trailing slash, so starting your string with a slash is not required try;
require_once(plugin_dir_path( __FILE__ ) . 'assets/MCAPI.class.php');