im having the same problem. Did you find any solution?
IAP testing behaves strangely when in beta/alpha. Once it's live it just works. I've spent a week trying to find the problem. There are posts on the internet from developers struggling to make it work, not just C2.
If you get the error, save yourself a time, test just if variables are triggered properly and once the game is live, make one purchase from a friend's device, just to be sure. Look at this, try testing it from a different device and account. There may be a delay with authentication, give it 48h and it might start working.