Installing CUPS service, how to access its config page?

[Bo Berglund]

I need to install cups on my Ubuntu 20.04 LTS server (no GUI) to replace an
older (I believe installed Jan 2014) non-functional printer server running on a
RaspberryPi2 which cannot be updated to a current version of the operating
system.

So I tried the instructions found here for Ubuntu:
https://ubuntu.com/server/docs/service-cups

I have done these steps:
- sudo apt install cups
- sudo nano /etc/cups/cupsd.conf
  Here I added the following:
  Listen 192.168.xxx.216:631 #My server's IP address so as to reach it
- sudo usermod -aG lpadmin <my user name>
- sudo systemctl restart cups.service

Now I wanted to configure the printers and the doc now switch to using a web
interface  so I tried to access the admin webpage on port 631, but since the
server is a non-GUI one I cannot run a browser towards localhost as suggested in
the doc:
http://localhost:631/admin

instead I tried this from my Windows10 using Firefox:
- http://ubuntuserv:631/admin
It shows this:

Forbidden
You cannot access this page.

Same if I try: http://username@ubuntuserv:631/admin

Apparently the Listen line in cupsd.conf did not help give access...

How can I solve this?
And is it possible to manage the printers from the command line of the server?

-- 
Bo Berglund
Developer in Sweden