Hi guys,
I have posted this in one other thread so I apologise if you see it twice, but this bug has been painful for me so I wanted to share it in both threads that showed up in my search.
I've been having this issue since last week and somewhat resolved it in my case, hopefully this helps someone else out.
After realising that the devices I was testing my IAP on were all running iOS 11 I tested some older devices.
It turned out, in my case, that iOS 11.0.3 was broken. I could make purchases on various versions of iOS 10 and today I updated my iOS 11.0.3 devices to 11.1 and all was working fine.
I'm sure I was testing on iOS 11 earlier last week or perhaps the week before but I honestly can't remember if that was using a beta version or something prior to 11.0.3.
I have reported a bug with Apple (35306392) just in case that helps.