I just got this to work (Dec 2023). I was experiencing the same issue that the OP posted about. Sadly the instructions in the post above did not work for me. The hard part for me was getting the token file. This only generates after you login.
I was able to get the token by running the application locally on my desktop, and moving the token manually to the docker container storage folder on my unraid instance.
After that I was able to run the sync command and kick off the backup!
Hope this helps someone in the future!
Instructions from the person who built the photo sync application: https://gilesknap.github.io/gphotos-sync/main/tutorials/installation.html#execute-in-a-container
The Github issue that helped me find my solution: https://github.com/gilesknap/gphotos-sync/issues/357#issuecomment-1159623596