Craft 3 not running on Ubuntu 18.04
When I had this problem previously, it was the permissions for the index.php that wasn't set to 644. 

Another issue I had once was my database password had some characters that wasn't accepted. Instead of getting errors, I just got a blank page saying Internal Server Error. Once I changed the password to omit weird characters, it worked perfectly. Wasted a ton of hours trying to figure that out.

Good luck!