Usually on a new install (Ubuntu) Apache2 Rewrite module is not activated.
To activate:
sudo a2enmod rewrite
… Enabling module rewrite.
To activate the new configuration, you need to run:
systemctl restart apache2
I also found in the documentation near the end of setup:
https://community.open-emr.org/t/manage-modules-internal-server-error/3326/16
Recommended Paste This to Bottom of Apache2 Config File
<Directory “/var/www/html/openemr”>
AllowOverride FileInfo
Require all granted
<Directory “/var/www/html/openemr/sites”>
AllowOverride None
<Directory “/var/www/html/openemr/sites/*/documents”>
Require all denied
BE SURE TO REMOVE THE QUOTES
INCORRECT - <Directory “/var/www/html/openemr/sites/”>
CORRECT - <Directory /var/www/html/openemr/sites/>
Took a few hours to track it down, perhaps the documentation could be noted to save others valuable time.