I was installing kde for my ubuntu 20.04 , and I was getting apt-get failed(100) error ,see image above.
I also checked I was able to install other application using tasksel successfully ,which means I was able to connect the Ubuntu update server.
The solution or workaround is to run apt update then try tasksel again.
sudo apt update
Above command works fine for me , I also search this issue in internet, if above method doesn’t work for you, you can try below command
sudo dpkg --configure -a
Or maybe need to check log file /var/log/dpkg.log to see if any other possibilities there.