I found that most answers are incomplete here. In case anyone else is still looking for this:
- Check if you have APP_KEY= in your .env, if not just add it without a value.
- Run this command: php artisan key:generate. This will fill in the value to the APP_KEY in your .env file.
- Finally, run php artisan config:cache in order to clear your config cache and recache your config with the new APP_KEY value.