How to test In App Purchases through Apple TestFlight?
TestFlight users don't require a sandbox account, but they will test against an automatically created sandbox account.
Sandbox Accounts
The moment a tester opens your application, a sandbox account is created for them. This means it's no longer necessary to create test accounts in iTunes Connect. Testing In-App Purchases was never easier.
Note that your TestFlight user is a legitimate App Store user. That user will be purchasing, restoring, etc. from the IAP in the Sandbox as opposed to the real IAP.
On iTunes Connect, the IAP can be "Approved" or "Ready to Submit" (or any other status, I think).
English
Testers - TestFlight - Apple Developer
Each build is available to test for up to 90 days, starting from the day the developer uploads their build. You can see how many days you have left for testing in the app’s details page in TestFlight. TestFlight will notify you each time a new build is available and will include instructions on what you need to test.
All in-app purchases are free during the testing period.
中文
测试员 - TestFlight - Apple Developer
从开发者上传的那一天起，每个构建版本可被测试长达 90 天。您可以在 TestFlight 的 app 详细信息页面上查看可供测试的剩余天数。
在测试期间,所有 App 内购买项目都是免费的.
Testing IAP is going to require a test user that you create under iTunes connect. Plus code in you app to implement IAP. Then to test it you "purchase" the item from within the app, under the sandbox, using that user id.
As far as I know TestFlight is no different than testing the IAP on your device by building directly to your device.
I'm assuming that you didn't just add the IAP bundle id and expect it to do something.
If you have not added code to support IAP, that answer is non-trivial within the scope of a SE answer. Google IAP tutorial. There are lots of good ones out there.