Reference: https://ubuntuforums.org/showthread.php?t=2282646

If you encounter such errors of fetching packages with apt/apt-get:

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/u/ubuntu-release-upgrader/python3-distupgrade_18.04.36_all.deb 404 Not Found [IP: 91.189.88.142 80]
E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/u/ubuntu-release-upgrader/ubuntu-release-upgrader-core_18.04.36_all.deb 404 Not Found [IP: 91.189.88.142 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Then do the following to give precedence to IP V4.
nano /etc/gai.conf
Scroll with arrow keys down to where it says
#precedence ::ffff:0:0/96 100
and remove the comment (#)
Press control+x to save, and you should be on your way.