We can't test In-App purchase in iOS14 gadgets as we can't make sense of the Sandbox account login in Setting Menu.
Beforehand it was there in Settings > Apple ID > iTunes and AppStore. In any case, in most recent adaptation (iOS 14), we can't discover the equivalent under Settings > Apple ID.
It would be ideal if you help ASAP.
In Settings, scroll down a bit and you'll see App Store >, just above Wallet & Apple Pay. The Sandbox sign in is now located in the App Store menu.
Hope this helps.