Ok… maybe I can help a little here.
The instructions you followed to do an installation were part of a thread I started when I was first trying to figure out the install problems. I stumbled upon another users set of steps and changed them up a bit to work for me “temporarily” util I could figure out a better way.
Well, I did figure out a better way and I even know why you are getting the “Sorry, we will be back soon” page when you try out your new installs. And it is mostly all fixed in my more recent tutorial.
Go here for the better instructions: (and read below about a new problem)
Okay, the instruction above are for a Debian v10 host, but will almost always work with Ubuntu v20.04 (but not newer versions).
The only thing that might be different on Ubuntu 20.04 will be the version of the MariaDB dataase that gets installed.
When I wrote the tutorial, Debian 10 was using MariaDB v10.3 but Ubuntu was using MariaDB version 10.5. To figure out what version MariaDB is installed during the setup, run the following command BEFORE you do the mysql setup step:
mysql --version
This will return the version of MariaDB as something between 10.3 and 10.6
If the version is 10.3 then proceed with the instructions as they are written. If the version is higher than 10.3 then you must SKIP the command that begins with:
UPDATE user SET plugin=
This step will generate an error with versions newer than 10.3 of MariaDB because it is no longer supported.
Once you get past that point then finish the instructions as written and you should end up with a fully functional ERPNext installation.
Remember…
The instructions I point to in this post only work with Debian 10 or Ubuntu 20.04 any other version of either OS will likely fail.
Hopefully in the next week to 10 days I can get some time to update the tutorial. Until then do as I described above.
Hope this helps. 
BKM