This error may be caused by Software Store having difficulty accessing what has been installed through flatpak.
You might try:
sudo apt install --reinstall gnome-software
Or even:
sudo apt install policykit-1
Only in regards to an issue related to the installation of Zorin OS Pro. The ZorinGroup consists of two people answering all help-requests on Zorin OS Pro installations. You may find the forum will give a faster response.