You can't build a Wither in Creative Mode.

When I try to build a wither, it doesn't work in creative or survival.

I was unable to replicate. Please post an image or a way to replicate this and or both.

Cannot reproduce

I am having the same issue but with the more recent snapshots. Wither spawns fine in 1.15 (and in 1.15 snapshots) but will not spawn in any of the latest snapshots on any difficulty or orientation.