Laravel-self-diagnosis is very useful to perform self-diagnosis test on laravel, it can check what you forget or miss errors, but on Mac, when you execute that you get the 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', ],