Are you facing the same error as this article's title? Then this article might help you out.

If you're developing applications using HTTP/SSL Protocols (regardless, you should!) on Laravel, by default, you'll be using Laravel's encrypter but in it's official documentation, it says:

Before using Laravel's encrypter, you must set a key option in your config/app.php configuration file. You should use the php artisan key:generate command to generate this key since this Artisan command will use PHP's secure random bytes generator to build your key. If this value is not properly set, all values encrypted by Laravel will be insecure.

Just execute the following command in your root directory:

php artisan key:generate

If the error still persists, try clearing the cache by doing the following:

php artisan config:cache
php artisan cache:clear
php artisan config:clear

Hope you found this article useful!