I've been encountering this problem as well. It seems like the SKTestSession just isn't being applied to the application when it's being run as part of a UI test. In Xcode 12.2, adding a 5 second delay before starting the SKProductsRequest no longer solves the problem. I know the delay worked at some point, but I'm not sure if this was on 12.1 or 12.0. Should I file a radar with a sample project?

I had this problem as well. Chrome and Firefox both gave me this error, but it uploaded without a hitch in Safari.