Mine trouble

[Chiprel]

Grey30 about your building problems.

You are unable to build on some biomes (mapple/autumn/puke forest for example)
You have to turn that terrain into dirt or pave it before building. Area might be flat but it just won't let you build if you wont turn it into dirt/pave.
As for smelter problem try paving a bit bigger area.
There are no problems if you try to build stuff on grassland. So if you are unsure. You might want to go on grassland, place a build sign in there and check building size with bumping into building area while you have leveling tool up(ctrl+L) to see its size in tiles.

[Grey30]

Try digging and then level it off with the same dirt you just dug.

Its working
TY