App Store No Longer Working in Mac OS 10.12.6 Sierra

I run Mac OS 10.12.6 on an iMac in order to continue using discontinued software. In the last week I started getting messages for some apps I purchased on the store when I only had the iMac telling me that I purchased the software on another computer and therefore in order to use it on the iMac I had to sign in with my Apple ID and password. Of course I use these apps with no problem on my Mac Studio that was purchased long after the iMac.

After proceeding to enter the requested information I receive the following message in a dialogue that appears:

"An unexpected error occurred while signing in.

The data couldn't be read because it isn't in the correct format.”

I signed out of the App Store thinking signing back in might sort the problem. Now when I try signing in I get the same error message.

I wiped the boot drive and restored it from an earlier backup. But it failed to solve the problem. I then ran the iMac from a bootable backup that was made when everything worked. But the same problem appears. Suffice to say when the iMac is booted in to a more recent OS like Big Sur I am able to run the apps and access the App Store without problems. But I need to run Sierra and want to run the apps that I paid for.

It may well be that the App Store is no longer compatible with this Mac OS. However it could also be that a particular certificate has expired and someone knows from where I can obtain a new one to get my apps working again.

I am not exactly happy that apps I purchased in good faith from the App Store are rendered inoperable because I run an older system. While it is perfectly understandable that if you run an old system you cannot expect updates, but to suddenly find that apps that worked perfectly well with the old system are now renedered useless is unacceptable. Software purchased outside of the App Store run perfectly well. Why shouldn't the same thing prevail with software purchased from the App Store? It does make one question whether it is better to purchase all software outside of the App Store if this is the end result.