linux-headers-5.4.0-56 needs to be reinstalled, but I can't find an archive for it
Just in case someone else ends up here. The solution to E: The package ubuntu-mono needs to be reinstalled, but I can't find an archive for it [duplicate] gave me something that helped me move the obstacle away:
sudo dpkg --remove --force-all linux-headers-5.4.0-56-generic
and then
sudo apt-get update