Laravel 5.5 with MySQL 8.0.11: 'sql_mode' can't be set to the value of 'NO_AUTO_CREATE_USER'

your laravel connexion (config / database.php) should be such that:

'mysql' => [
        'driver' => 'mysql',
        'host' => env('DB_HOST', 'localhost'),
        'port' => env('DB_PORT', '3306'),
        'database' => env('DB_DATABASE', 'forge'),
        'username' => env('DB_USERNAME', 'forge'),
        'password' => env('DB_PASSWORD', ''),
        'charset' => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix' => '',
        'strict' => true,
        'engine' => null,
        'modes'  => [
            'ONLY_FULL_GROUP_BY',
            'STRICT_TRANS_TABLES',
            'NO_ZERO_IN_DATE',
            'NO_ZERO_DATE',
            'ERROR_FOR_DIVISION_BY_ZERO',
            'NO_ENGINE_SUBSTITUTION',
            ],
    ],

Add the following on each of your MySQL connections:

'modes' => [
     'ONLY_FULL_GROUP_BY',
     'STRICT_TRANS_TABLES',
     'NO_ZERO_IN_DATE',
     'NO_ZERO_DATE',
     'ERROR_FOR_DIVISION_BY_ZERO',
     'NO_ENGINE_SUBSTITUTION',
],

The next release of Laravel 5.5 will add support for MySQL 8.0: https://github.com/laravel/framework/pull/24038

UPDATE: Laravel 5.5.41 has been released.