So I made a 40x40 lot and started to build on it. About halfway through I started getting these wall requires a flat base messages and then it stopped me building walls there. So I thought flattening the lot would fix it.. but it didnt. Now i have no clue what to do and im kind of in panic mode.. D:

http://i54.tinypic.com/2yooep5.png

Building questions go under Create --> Building. Thread moved.

Flatten it manually via the terrain tool? You may need to remove some walls to do the flattening

If all else fails, since you already have a picture of the layout, just start over but flatten the Lot first this time.
Everybody does this, I started over countless times when I get that pesky floating ceiling problem or the railing not attaching to stairs problem, after the whole House had been finished and furnished...etc. No need to panic.
Just chalk it up to lessons learned and when you remember it, you don't do it again.

Or use the cheat "constrainFloorElevation False" to allow you to flatten the lot then turn it off "constrainFloorElevation True" and continue?

Just an option!