Error signing into App Store.

See if anything here helps:

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

Typically if your computer cannot run Big Sur you will get a clear error message about that, though conceivably that could be generating that message.

If you can't resolve this, call Apple and ask them since it is hard to do anything with such a generic message.

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232

Hi Christiekhall,

Welcome to the Apple Community, where some of us do our best to help each other :)

It seems that not all Macs can / do update from the App Store.

First you might want to check the model (Year) of your MacBook Pro

Once you check the Year (from "About this Mac"):

You can scroll down to the correct MacBook Pro Model,

and find out which versions of the Mac Operating System are suitable for your Mac.

Here's the link to help you:

Identify your Mac Book Pro model

https://support.apple.com/en-us/HT201300

If your Mac isn't compatible with the latest macOS, you might still be able to upgrade to an earlier macOS,

such as macOS Catalina, Mojave, High Sierra, Sierra or El Capitan

If you can update to the latest (Big Sur) version: here's how

https://support.apple.com/macos

OR

How to get earlier versions of macOS

https://support.apple.com/en-au/HT211683

Hope this helps you :)

PS

Re: Signing into App Store

Here's a discussion thread with good troubleshooting info:

https://discussions.apple.com/thread/252755756

OR

If you have changed your Apple ID or password lately you might need to update your sign-in information with any Apple services (Eg App Store) that you're using. The support page below can help in this situation:

What to do after you change your Apple ID or password:

https://support.apple.com/en-au/HT204071

Cheers :)