You does not find it because that old version is now gone !!! Synaptic can list the available active versions for sure.
https://launchpad.net/ubuntu/+source/linux ** Changed in: update-manager (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/1770936 Title: Archive "linux-image-4.13.0-21-generic" not found To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1770936/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
