making a basement

Could you please post a screenshot of the house/basement?

I don't know how to take a screenshot in the game. Can you tell me how?

bev6usa​ Generally, when you can't place a stair, the issue is usually that there is something in the way on the floor above blocking the upper landing from placing.  You must have at least one tile clear at the top of the stair to place the stair.  So, check the floor above in the area where the upper landing of the stair will go and move anything in the way.  If there are any walls in the way, remove them.  Then try to place the stairs.  

Hope this helps.

When I try to place the stairs, I get it can't be placed in the terrain

bev6usa​ From what you've said, I don't understand what you are trying to do other than place a stair.  A pic would be very helpful right now.  To take one depends on your platform so what platform are you playing on?  I play on PC and to take a screenshot, press and hold the Windows key and press print screen.  For a snip, the combo is Windows key, shift-S.  Idk how you take screenshots on Mac or the consoles.  You can also use your phone and transfer the pic to your computer. 

Hope this helps.

I got it figured out thanks for trying to help me