So with IOS 11, sandbox purchases is just totally broken.
There's a thread from June first reporting this problem https://forums.developer.apple.com/thread/80560.That was 4 months ago and still the problem persists.
I have several devices with iOS 11 installed on them, I also have several existing sandbox test IDs and have created several new ones to try to get things to work.
Not one single combination of device with existing id nor new id works. Every time the attempted purchase fails. Nor does the workaround provided in the thread above work.
*** 100% failure rate with iOS 11***
I still have one device with iOS 10 installed, *every* single combination of purchase with every sandbox id works.
***100% success rate with iOS 10 using the exact same app and product ids and apple ids as used when iOS 11 attempts fail ***
That tells its own story. Need I say any more, except to ask and plead when will this be fixed?