Comment 2 for bug 1955611

Thank you for taking the time to report this issue and helping to make Ubuntu better.

This package failure looks like it was caused by bad ISO download, corrupted install media, or device failure, assuming /dev/sdb is your installation media; the error "/dev/sdb: No medium found" is an ability to recognize the media as valid. The question to attempt to download is the solution offered in that circumstance.

Possibly useful : https://tutorials.ubuntu.com/tutorial/tutorial-how-to-verify-ubuntu#0 though it sounds like that was not your issue (having performed checks), but the writing of the ISO was not done correctly (possibly with older software that couldn't cope with 20.04.3 media).

For Support, this bug report can be converted into a question, or you can ask a question on https://answers.launchpad.net/ubuntu. You can also find help with your problem in the support forum of your local Ubuntu community http://loco.ubuntu.com/ or asking at https://askubuntu.com or https://ubuntuforums.org, or for more support options please look at https://discourse.ubuntu.com/t/community-support/709