What about the Action "IAP.PurchaseProduct"? Does it trigger the Buy dialog box?
The "On Purchase" is only triggered after a successfully purchase that is initiated by the above action call.
Next, how are you testing your app on the device? TestFlight (using Production certificate) or Adhoc test (using Adhoc provision certificate)?
If Adhoc, then I think the IAP will not work?
Next whats the Status (eg: Ready for Review, Approved, Ready for Submission, etc) of your IAP in iTunes Connect?
Lastly, whats the status of your iTunes Contracts? If Financial Contracts are not effective, then IAP will not showup in the game.