When attempting to access the Notes page of a course I was encountering the error EdxNotesServiceUnavailable. I was able to confirm that the Notes service was running correctly and that I was able to create new notes in a unit as well as fetch them from the database. What I found was that the Notes page was hitting the internal API while requests to get and create notes on units were hitting the public API. When running Edx Notes in Tutor dev I found that the service was on port 8120, but the internal API was configured to use port 8000 which is what was causing the service to not be found. Changing the internal API port locally fixed this issue on my machine.
I have opened a pull request to the tutor-notes repository to address this issue.