OK,
So I span up a AWS lightsail instance with Ubuntu 18.04 and followed the guide you referenced EXACTLY.
So, to troubleshoot, we need to replicate the issue. If there is an issue with Ubuntu, Nextcloud or the guide you used we should encounter the same error.
I completed the install and ended up looking at the setup page: http://3.8.127.87/nextcloud/
There was ONE correction I needed to make. To this end can you post the output of:
1.) apachectl configtest (when run from terminal).
2.) The exact contents of your /etc/apache2/sites-available/nextcloud.conf