Hi everyone -
I made an account here to share with everyone how I solved this issue. I was getting the same error and spent an evening trying to figure this out lol.
First, SSH’d into your Synology
Then run the command ‘sudo tailscale up’. Terminal then returns a custom URL which I was able to paste into my browser where I was allowed to log into my Tailscale account. From there everything was authenticated and is now running as it should on my Synology.
Hope that helps someone!