The error you are encountering is correct and expected behavior. You are attempting to release-upgrade to a release that is already past End Of Life (July 2021); those repositories were withdrawn.
There is no constant, tested migration/upgrade path from an LTS release to a non-LTS release. When the calendar is unfortunate (like now) the tested and supported method is to backup your data and reinstall Ubuntu.
FYI: The next 6-month release, Ubuntu 21.10, is scheduled for release in a mere two days. The release you are attempting to install, 21.04, will reach End of Life in a mere three months.
Advice:
- Backup your data.
- Create an Ubuntu 21.10 LiveUSB installer.
- Install 21.10 over 20.04 without formatting. The new version of Ubuntu will overwrite the older while leaving your data and customizations unaffected.
- The backup is in case you make the wrong selection and format instead.
- Note: Some folks who migrate from LTS to non-LTS releases are doing so temporarily for testing; seeing if hardware or software work with the newer release. You DON'T need to commit to the newer release -- you can use a LiveUSB's "Try Ubuntu" environment to conduct accurate, reliable testing while leaving your LTS system untouched.