There is a requirement on MacOS Mojave to enable accessibility in order for this feature to work.
There should be an automatic pop-up requesting permission when scanning the first time after clicking on the document button.
If you don't see the prompt, please follow the steps below to enable this manually in the Privacy & Security section of System Preferences:
1. Open System Preferences
2. Click on Privacy & Security
3. Click on the Privacy tab at the top
4. Choose Accessibility from the list on the left
5. Click the lock icon to make changes
6. Check the box next to Scanmarker.