Testing IAP will require the use of a test user under iTunes connect. Plus the code in your application for implementing IAP. Then, to verify this, you “buy” an item from the application, in the sandbox, using this user ID.
As far as I know, TestFlight is no different from testing IAP on your device, building directly on your device.
I assume that you did not just add the IAP packet identifier and expect it to do something.
If you have not added code to support IAP, this answer is non-trivial as part of the SE response. Google IAP Tutorial. There are many good ones.
badweasel
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