Welcome to the Ubuntu MATE community!
What output of cat /etc/os-release and grep ^deb -r /etc/apt/ --include=*.list do you have?
On current stage you can run the upgrade from 20.10 to 21.04 by specifying -d flag in
update-manager -cd
or
sudo do-release-upgrade -d
After upgrade you can install all new packages with
sudo apt-get update
sudo apt-get dist-upgrade