There is a function in SoreKit AppStore.showManageSubscriptions(in: scene) this shows a list of Sandbox purchases. There you can also cancel and switch the subscriptions

I've got the same problem. Error messages are only available in the Report Navigator (cmd + 9). Which is very uncomfortable to work with and features like "fix" or "fix all" don't work. Tried 14.0.0, 14.0.1 RC, 14.1 Beta 2+3.