If you are trying to RESTORE an app or subscription purchase, and you see a message something like
Regrant Failed! Please Purchase Software Application First
this usually indicates that the iTunes account you are using is not the one which made the purchase.
Were the app and Pro subscription purchased under the same iTunes account which you are currently using on your device?
The purchase of a Pro subscription is via an in-app purchase, which means that the purchase of the pro subscription is tied to the Apple iTunes account used at the time it was purchased.
If you subsequently change the iTunes account used on your device, there is no way for the Apple server to verify the purchase against the current account, as that purchased is not "owned" by the current account.
If you haven't changed iTunes accounts, try logging out of your iTunes account under your device settings (not the app settings), and sign in again, then try the "Restore" option again.