Could You open the Terminal and run sudo apt update && sudo apt upgrade there. Because in the Terminal You should get further Output than with this Message. And when You have that, post please all of it here.
Another Thing: Do You have choose ''Main Server'' as Source in the Settings of Software & Updates?
Open the Software Updater and click on the Stopp Button. Then click on Settings.
Open the Gnome Software Center. Click there on the Settings Button beneath the min/max/close Button and choose the Option ''Software Sources'':
Please don't be irritated by the Language. My System is in german.
When You go in the App Grid, you should find a Symbol for Software & Updates.
Hmm ... for me it looks like that something is broken. What did You before this Problem appears? Did You install Software, made an Update or something?
Yes, I think I change the settings. Like nvidia-driver-535 to nvidia-driver-545. After that I booted the ZorinOS in advanced options. Now I am using in the old boot selection.
I didn't remove just change the select option but after that I can't boot. I can use safety mode and the older version select in the advanced options for boot screen.
Then it would be good to remove the Driver. So, open the Terminal and type sudo apt remove nvidia* libnvidia*
That will delete the Nvidia Drivers. After that install the Nvidia Driver again with sudo apt install nvidia-driver-535 for the 535 Driver or type sudo apt install nvidia-driver-550 for the newer 550 Driver.