Our apologies for the delay. We had a planned maintenance in sandbox environment on 15th June, so this is most likely why you were unable to view your REST app credentials. We are actively working on plans to migrate the PayPal legacy sandbox environment to a new data center. As part of this initiative, we scheduled several maintenance windows, where you may experience issues in sandbox environment. You can find more details about this in our sandbox migration bulletin.
That being said, if you're still receiving this error in PayPal Developer Dashboard, feel free to reach out to us and we'd be happy to investigate.
... View more