You're likely to encounter this while running
sudo apt update && sudo apt upgrade
but you may encounter this more when you upgrade to Zorin 17 (Ubuntu 22.04). For example:
The following packages have been kept back: gnome-shell
I've also seen other people report this.
The error is likely because the dependencies have been changed. To fix this, run
sudo apt install --only-upgrade
then the name of the package. For example:
sudo apt install --only-upgrade gnome-shell
Hope this helps!
Reference: [Fixed] "following packages have been kept back" Error in Ubuntu