Laravel-self-diagnosis is very useful to perform self diagnosis test on laravel, it can be check what you forget or miss errors, but in Mac, when you execute that you get error, following the steps to solve it.
For you started, you need to know what mac OS version do you have:
$ php -r 'echo PHP_OS;' Darwin%
slef-diagnosis.php changes to
'required_locales' => [ // 'en_US', // 'en_US.utf8', PHP_OS == 'Darwin' ? 'en_US.UTF-8' : 'en_US.utf8', ],