I see a similar error message. I had the same scheme working fine some months ago in May 2025, with Xcode 15. Now with Xcode 16.4, on [productsRequest start] I get the mentioned error in the Xcode log and the delegate returns my identifiers as _invalidIdentifiers. Could it be an Apple bug?
I can fetch a new synchronized StoreKit configuration file with up-to-date data. So my environment seems not altogether wrong.