It is not sufficient to just disable the "Other software" as you will still have package version from other repositories, like the deadsnakes PPA, which may provide package versions greater than the release to which you are upgrading. Subsequently, you'll need to reenable the other software sources and then use the tool ppa-purge to remove packages from PPAs.
** Changed in: ubuntu-release-upgrader (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1970018 Title: Cannot upgrade to ubuntu 21.10 from ubuntu 20.04. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1970018/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs