The latest version of Panels 2.15.0 contains a bug that makes the app unable to validate purchases.
We have fixed it 1 hour after the app was released to the AppStore and stopped the roll-out.
A new version 2.15.1, fixing this issue, is already in Review and should be out in the next few hours.
Sorry for the inconvenience.
In the meantime, we recommend not updating or using the latest Testflight