Hey there, let's work through this upgrade issue step by step! The 404 error you're hitting for libssl1.1 is a typical problem when dealing with an end-of-life Ubuntu release—your system is trying to fetch packages from the impish-security repo, but Ubuntu 21.10 (Impish Indri) reached its end-of-life in July 2022, so those official security repositories have been archived and are no longer accessible.

Here's how to fix it:

• Update your software sources to use archived repositories
  The old Impish packages are now hosted on Ubuntu's old-releases server. You'll need to edit your sources list to point to this location:

  1. Open the sources list file with a text editor (we'll use nano here for simplicity):

     sudo nano /etc/apt/sources.list
     

  2. Replace every instance of http://security.ubuntu.com/ubuntu and http://archive.ubuntu.com/ubuntu with http://old-releases.ubuntu.com/ubuntu/. For example, a line like:

     deb http://security.ubuntu.com/ubuntu impish-security main amd64
     Should be changed to:
     deb http://old-releases.ubuntu.com/ubuntu/ impish-security main amd64

  3. Save your changes by pressing Ctrl+O, hit Enter to confirm, then exit nano with Ctrl+X.

• Refresh your package cache
  Now run the update command to let your system pull the latest package information from the archived repo:

  sudo apt update
  

  This should resolve the 404 errors since we're now pointing to a repo that still hosts the old Impish packages.

• Retry the upgrade
  You can now go ahead and run the upgrade again. If the previous upgrade was interrupted, use the --fix-missing flag to help resolve any lingering package issues:

  sudo apt --fix-missing upgrade
  

• Upgrade to a supported release (critical for security)
  Once you get the current upgrade working, it's strongly recommended to upgrade to a supported Ubuntu release like 22.04 LTS (Long Term Support). End-of-life releases don't get security patches, which leaves your system vulnerable. To upgrade:

  sudo do-release-upgrade
  

  Follow the on-screen prompts to complete the upgrade to the latest supported version.

If you still encounter issues after these steps, double-check your sources.list file for typos—even a small mistake can break the repo connection. Also, ensure you have enough free disk space to complete the upgrade process.

备注：内容来源于stack exchange，提问作者Run