This error displays when you move the lower boundary of a room above the computation height, or when you adjust the computation height of a level below the lower boundary of a room.
Error: Room’s lower offset is above the computation height.
Solution: In the error dialog, click Cancel. The position of the lower boundary of a room must be below the room computation height of the level. To adjust the position of the level's computation height, select the level and change the Computation Height on the Properties Palette. If this error occurred when changing the computation height of the level, click Adjust Limits to change the lower boundary of the room to match the position of the computation height.