If you see this error you are missing APP_KEY.
Make sure you have .env in your main directory.

If you don’t have it one yet, create it.

root@vmware:/srv/www/laravel/public_html# > .env

Generate key

root@vmware:/srv/www/laravel/public_html# php artisan key:generate
Application key [base64:i7QndWbN33zY1x013Yw2cju9KQsxpT/1nnf8/3ziZ+U=] set successfully.

And now just add it into your .env file, clear cache and you are good to go

root@vmware:/srv/www/laravel/public_html# cat .env
APP_KEY=base64:i7QndWbN33zY1x013Yw2cju9KQsxpT/1nnf8/3ziZ+U=
APP_DEBUG=true
APP_ENV=development
APP_URL=http://laravel

root@vmware:/srv/www/laravel/public_html# php artisan cache:clear
Cache cleared successfully.