If you can't even turn on Email to GoodNotes feature, please open your iPad Settings (not GoodNotes Settings), tap Apple ID > iCloud, and turn on both "iCloud Drive" and "GoodNotes". This is because the feature requires an iCloud account.
If you can enable the feature, but immediately see an error message that says: Unable to create email address. Please try again. If the problem persists, contact our support., it's likely due to a temporary issue on our backend, so please try again later. We're taking steps to prevent this from recurring. For us to verify this, please send us the Diagnostic Data. Please open GoodNotes ⚙️ menu in the library view > About > Export Diagnostic Data.