Thoughts, ramblings and rants…
The standard Ubuntu versions go EOL quite quickly and it’s easy to miss the upgrade window such that running do-release-upgrade yields:
An upgrade from 'groovy' to 'impish' is not supported with this tool. In this example, Ubuntu is looking to upgrade from 20.10 to 21.10 skipping 21.04 which is not supported. You’ve probably also reached a situation where you cannot even upgrade your current packages as the repositories have also been EOL’d and do not exist.