There is million things that can go wrong... I would start by log in into your webserver instance and checking out some logs and see what’s written in there.

/var/log/https or Apache or Nginx

Next check the laravel logs

[app root]/storage/logs/laravel

Post your logs if you need more help. 500 error means your app is crashing this could be a configuration issue, permission issue or something wrong with your DB credentials.

Follow this TUTORIAL it should help you create the LEMP environment you need for Laravel

This is why it is easier to host your Laravel website on aws using platform, like Cloudways Laravel hosting. Using this platform, you can host your Laravel application on aws in just one-click. Since the servers are managed, if you face any issues, like the one you mentioned, their support team is there to resolve it. Directly hosting with aws means you won't get that kind of support.