I have seen some older posts, none of which really seemed resolved, so I'm going to give it a current shot.
I bought a new Raspberry Pi yesterday, I have connected to it via SSH and set Teamviewer up using the instructions from https://pimylifeup.com/raspberry-pi-teamviewer/
I have added the Teamviewer ID to my Computer & Contacts and it shows as available, and when I power off the Raspberry Pi it shows as unavailable, so that portion is working it seems.
When I try to connect to the Pi, it just says Connecting in the bottom left for several minutes and eventually shows the error in the bottom right of "Connection not established. Authentication Canceled"
Any clue how to fix this?