If “Link with iCloud” option is disabled in Due > Settings > Sync, that means Due has no access to your iCloud Drive.
This can be because:
- iCloud Drive is disabled on your device, and/or
- Due has been explicitly excluded from using iCloud Drive.
Please first check that iCloud Drive is enabled on your device with the steps given by Apple. Then, ensure that "Due" is enabled for use with iCloud within the same settings page.
If your settings are already correct, please restart your iPhone or iPad.