If you are receiving the message, "The site is currently offline due to encountering an error. If the problem persists, please contact the system administrator.", most likely it is because of a permissions issue. Try the following:
On Ubuntu, Apache usually runs as the www-data user. Some implementations of Apache run as the apache user.
Try the following, swapping out /path/to/mautic for your actual Mautic folder name.
chown -R www-data:www-data /var/www/html/path/to/mautic
This should resolve the 500 error, then for good measure you should set these directories as writable.
First change directory to your Mautic folder then run the following commands.
chmod -R 755 /app
chmod -R 755 /media
chmod -R 755 /translations