Whenever I try to place a wall or floor or door or anything to make a house... it always says that..

How do I do that

As long as you built a foundation and floor, you will have no problem putting on walls. But if you're building another floor on top, say 1st floor, walls can only be erected if there is a wall directly underneath. Say you're trying to make rooms on first floor, you first need to make temporary wall underneath on the ground floor, after erecting your wall upstairs, you can then destroy the wall underneath. The floor and wall on 1st floor will not fall down. With this skill, you can also make a floating house

I tried this and it still didn’t wouldn’t work... am I doing something wrong?

I don’t know if your still playing this game or not lol but if you are I figured it out you have to go in the craft menu and craft a wooden foundation and then you gotta place the wooden floor on top of it(you need a foundation every time) hope this helped