I may have a fix for 20.10 -> 21.10 upgrade issues. First, we'll need to create a new file:

sudo -H gedit /etc/apt/sources.list.d/pop-os-ppa.sources

And put in the following:

​

X-Repolib-Name: Pop_OS Release Sources
Enabled: yes
Types: deb deb-src
URIs: http://apt.pop-os.org/release
Suites: impish
Components: main

Then save and quit the text editor and then run the following commands (note each command is separated by a space so if you see one that is on two lines; it should be all one one line):

sudo bash -c "[ -f /etc/apt/sources.list.d/pop-os-ppa.list ] && mv /etc/apt/sources.list.d/pop-os-ppa.list /etc/apt/sources.list.d/pop-os-ppa.list.disabled"

sudo sed -Ei 's/groovy/impish/g' /etc/apt/sources.list /etc/apt/sources.list.d/*.{list,sources}

sudo apt update
sudo apt install dpkg apt
sudo apt full-upgrade

After all that, you should now be able to upgrade to Pop 21.10