When SSH clients first connect they ask the server for a pseudo terminal (PTY), to enable terminal colors, clearing the screen etc.

In this case the SSH server declines that request because it simply wants to send back one line of text. The client falls back to text mode and works fine, but issues the warning anyway. You can suppress it with `ssh -T mars.vtllf.org` like I did in the demo video.

On the surface, it means SSH tried, and failed, to initiate an interactive session. In context, it means that either the developer has disabled new users, or this is simply broken at the moment.