Upgrade Ubuntu 20.10 to 21.04

Question

I try to upgrade my Ubuntu 20.10 to 21.04.

1. Using the GUI Software Updater gives following information:

   Software updates is no longer supported for Ubuntu 21.10 To increase security please install Ubuntu 21.04

   So, I follow instruction and click the Upgrade button! The GUI Software Updater just shuts down without response or information.

2. I tried the release upgrade as below.

   $ sudo do-release-upgrade
   [sudo] password for thomas:
   Looking for a new Ubuntu release
   Your Ubuntu release is no longer supported.
   For upgrade information you can visit:
   http://www.ubuntu.com/releaseendoflife
   Install all available updates for your version before upgrading.*
   

   As far as I known I have installed all updates from Ubuntu, and the GUI Software & Updates app is configured for daily information for updates. All previous version updates without problem.

So I kindly ask for your help in this issue to upgrade 21.04.