Can it build OK in other spots? It might be the where that’s the problem.
Generally speaking hearthlings need two tiles of walkable space around the base / perimeter of any given structure in order to build it, and if it isn’t there, they’ll get confused at some point in the scaffolding and stop. (You can get away with less sometimes, but that’s the safe margin).
The workaround is to build a two tile wide platform around your structure first (perhaps on separately built column supports) then deconstruct it after you finish building the main house.
You can build a lot of things in this game but the AI has some specific things that short circuit it and they aren’t always obvious.