I've only ever used it as a consumer (to get code from a repo) rather than as a VC system. By the time I finally got my head around VCs git was all the rage so I've never really used it...

Having said that, you are completely right. The idea is to make this stuff easy. So it's either a bug or it needs to be documented (and it's a bug that it's not already documented! :)

I just had a quick google and the error suggests that it has authenticated you fine, just that the user you used doesn't have authorisation to push to the repo (which suggests a bug to me). I don't have the appliance running and handy so I can't test but this stackoverflow answer sounds like it might be useful (even though TurnKey is Debian based I would imagine that stuff like this would be fairly consistent across Linux OS (although maybe some of the paths may not be quite the same).

Let me know how you go and perhaps we can create some docs on this from your investigations. From what I google suggests read a quick and dirty way might be to conf/svnserve.conf and set anon write access (although obviously only good for testing.