basement
I tested this and it didn't happen for me. I was able to draw walls on the basement level with both the wall and room tools with no problem or error message. The only time I saw the message was when I moved the tool pointer outside of the lot boundary. I do run on a PC though so it may be Mac specific but idk.
Are you able to get a basement drawn using the basement wall and room tool? If you can, then that may have to be your workaround if the problem persists.
A couple questions though.
Have you tried repairing the game?
Have you tried deleting the localthumbcache file?
Have you tried creating a new game (separate Sims 4 folder) and checking to see it happens in that save? Might want to try it twice with and without mods\cc.
I hope this helps.
