If you can’t place an object on the floor, make sure that this floor suits the object. Some of them require floor made of planks or stone.

And also don't forget to check the maximum number of the object or structure that can be placed on the base, the limit can be seen during the placement of the object in construction mode, at the bottom of the screen. The same object is probably already exists at your base, so the game thinks that you don’t need another one at all :)