If you receive an error message when trying to export to Dropbox, GoogleDrive, OneDrive or Box, please first make sure that your device has an active and working internet connection.
If the error still occurs, please relink your cloud storage account in GoodNotes under Options>Settings>General, by toggling the switch off and on. You should automatically be redirected to the cloud storage and will have to grant permission.
The file you're trying to export/import might be too large to be handled by the cloud storage. Please try exporting/importing it via iTunes instead by connecting your device to your computer with a USB cable.