Last time when I was making update for my Ubuntu 20.04 I got the following problem:
Err:2 http://pl.archive.ubuntu.com/ubuntu focal-updates/main amd64 libc-dev-bin amd64 2.31-0ubuntu9.2 Temporary failure resolving 'pl.archive.ubuntu.com'
The strange thing is the http://pl.archive.ubuntu.com/ubuntu link is working.
I had the same problem. There was a problem with the server. In my case, the solution was to remove
pl. from URL from
Edit apt source list with:
sudo nano /etc/apt/sources.list
deb http://pl.archive.ubuntu.com/ubuntu/ focal main restricted
deb http://archive.ubuntu.com/ubuntu/ focal main restricted