I’m seeing this exactly issue as well. In app product has been visible and returns all the required data (price, etc) but try to purchase from an app in TestFlight gets the same error “Item Unavailable”. So it’s not just you.

It seems to be slow propagation as the returned products var is now populated, but doesn't allow a purchase yet, so I'm guessing it's just slow. I haven't heard back from Apple as yet.

For what it's worth you're not the only one having this issue and I've spent hours trying to debug it and ended up opening a case with Apple to find a solution. My app isn't yet live so I thought it might have been a sandbox/testflight issue.