Level the ground. I think the smelter and kiln can't be placed on a wood floor. Improvements have to be close enough to their related bench.

The improvements are quite bothersome as they have to be close enough to their respective bench, BUT also far enough away from each other... I often hollow out a large area under the floor to place most workbench / forge attachments and leave just a few choice ones above to decorate the workbench / forge area, such as tool racks.

The Hoe comes in handy to level the ground.
Try that first.

its clipping into something. Usually its pretty forgiving, but at times, there is a pebble in the way or something. Try smoothing the land a little more.

TY for the reply I will try that the next time I play.

Yes, the bench add-one have certain space requirements. It is a minor annoyance.

However, I am generally good with how forgiving the clipping is. You want to put your smelter in your base? No problem. Build the base, knock out the lower walls and floor planks in the corner, then drop the smelter halfway into the room. Now you can replace the wall and floor blocks by clipping them into the smelter. This can be done with a lot of items. Some games you cannot do this at all.