If you can't even turn on the Email to Goodnotes feature
Make sure to turn on iCloud sync as this is required.
If you can enable the feature, but immediately see an error message
The error message might say: 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: open GoodNotes ⚙️ menu in the library view > About > Export Diagnostic Data.