red means it lacks support.
2 walls high on 3 sides and apply a couple of roof tiles until it is happy and lets you make stuff.
some places you can just use poles to put up a roof, but the bench specifically needs walls too

here is a slightly more than you need example (I have a light and a chest and flooring in my shed, but the general idea is easily seen:

https://steamcommunity.com/sharedfiles/filedetails/?id=2641255991