Sent a bug report. Found an interesting (for server specialists) line in my log: "IDSService send request failed. Error: Error Domain=com.apple.identityservices.error Code=7 "Could not find valid account" UserInfo={NSLocalizedDescription=Could not find valid account}» This seems to explain the reason why we can no longer buy/test our IAPs.

Update to iOS 14 solve problem, but, how about 12, 13? Apple, please, do something. Can anyone answer, do you using the StoreKit directly, or are you using something like SwiftyStoreKit?

i am facing the same issue

Caught this error about 2 days ago and still continue catch it now, all my IAPs are fell off together for one app which I testing before send to review. Creating new accounts for sandbox environment didn't give results. In general, now, when I trying to buy an in-app purchase, the system asks for a password twice and gives an error with StoreKit error.code = 0. Tested on iOS 12.4, 13.7 with Xcode 12.0.1 Sent mail to Apple support, but no replay yet.