The format for an IF formula is:
Code:
IF ( condition , <result if true > , <result if false> )
In embedded IF's, the <result if false> part is another IF. Therefore, the last IF, because it is the last one, must have a <result if false> that is a value.
Look at my example in my previous message:
Code:
IF(RoomType=0, Area/300, IF(RoomType = 1,77, IF(RoomType = 2, 50,100)))
The number '100' is the last <result if false> part that I am talking about. Since this is the last IF, you need to tell Revit "otherwise, do this". You're missing that part in your formula. (Without looking at other issues)