Background
You were refunded for the app, but have changed your mind and want to repurchase it. However, attempting to do that might give the following message:
Solution
It's likely an App Store bug. Please do the following:
1. Open the Settings app > iTunes & App Store.
2. Tap your Apple ID and choose Sign Out.
3. Visit GoodNotes App Store page and choose Buy.
4. Choose "Use Existing Apple ID" and sign in with your Apple ID.