One of my TestFlight users just got this message as well, except the error / status code came up 21105 (which apple vaguely describes as an internal data access error). The error: "There's no information available for In-App Purchases. Try again later." 21105 [Environment: Sandbox].
I've repeated his process on my end (register for account, then auto-renewable subscriptions screen pops up) and everything works as expected. No errors and I'm able to complete purchase with TestFlight sandbox account.
Things I've had my tester try:
Delete, re-download app from TestFlight and retry. Result: Same error 21105
Sign out of their iTunes account, Open app and retry registration. Now app requests Apple ID and password. User enters credentials. Result: Same error
Not sure how to get this fixed as it seems to only be happening to this particular tester for now. I've sent emails to my other testers to check for this error and will keep everyone posted here.
Anyone else coming into this issue and/or have fix suggestions?