Treeguards will not spawn

This is not a bug, this is as designed, especially if you're ruins rushing early on in the game. The Wiki says you've got a 1.33% chance of spawning a treeguard with each tree chopped. That means you'd have to chop 100 trees to maybe get one.

I've been there when ruins rushing trying to get the moon staff before day 11. Sometimes I've gotten one, most of the times I have not. Best bet is to try and chop/dig the living log setpiece. 

you need to change your tree guard spawn rate to get more frequent spawns. it is located at the bottom of the mob settings page with the other bosses/minibosses