Could not download the upgrades for version 20.10 and cannot see version 21 on the updater [duplicate]

Solution 1:

It is normal that you cannot, at this time, update from the LTS release 20.04 to the current interim release 21.04. That is because 20.10 currently is not anymore supported, whereas in Ubuntu, you can only upgrade from one edition to the next.

Long story

You need to know that Ubuntu only allows upgrades from one version to the next. And when you upgrade, both the old and the new version must be within the support period.

From 20.04, you could only upgrade to 20.10. Alas, 20.10 is not anymore supported. So you cannot anymore follow the upgrade path.

20.04 is a release with long time support, an "LTS-resease". Therefore, an upgrade to the next LTS release will become available when the next LTS version appears. That will be version 22.04, next spring. So at that time, you will be able to upgrade your current LTS 20.04 to a newer version. For now, your 20.04 version is fully supported.

When using an LTS release, your system is supported for 5 years, and you upgrade only every two years. Ubuntu has also so called "interim releases". These have only a nine month support cycle. If you jump on the interim releases, you need to upgrade every nine months.