Apple acknowledged the issue: https://www.apple.com/support/systemstatus/
Edit: Some people have reported this issue on iOS 15.7, and the Mac App Store as well, so this seems like it is not related to the new update.
Edit 2: Looks like Apple has fixed the issue (not reflected on the System Status page). Other users have reported that restarting their devices helped fix the issue
I was experiencing the same problem: unable to purchase or update any items from the App Store, with an error code (13).
I came across another thread here - Cannot Install or Update Apps in App Store - Error 13 app-store-error-13 - which basically suggests deleting a cookie-preferences file for the App Store. I tried it, and it indeed worked for me.
The steps are:
1) quit the App Store app, if it's open.
2) delete the file, which will be in: ~/Library/Cookies/com.apple.appstore.plist
3) restart the App Store app, and try purchasing or downloading/updating something.
Let us know if this fixes the problem for you! :-)
I reinstalled OS, because i don' t have any other ideas.