On Update Field - IIF Function

Hi there,

I have added the 'On Field Update Set' user property in Opportunity BC with following definition:

Name: On Field Update Set 36
Value: "Sales Stage", "Opportunity Status",
"IIF (Left([Sales Stage], 1) < ""3"", LookupValue(""REASON_WON_LOST"",""Active""),
IIF (Left([Sales Stage], 1) = ""3"", LookupValue(""REASON_WON_LOST"",""Won""),
IIF (Left([Sales Stage], 1) = ""4"", LookupValue(""REASON_WON_LOST"",""Lost""))))"

But it seems that it is not working.
Can anybody please tell me if anything is wrong in the above definition and what should be the cause why it isn't working?

Is it possible that three options are not supported?

Many thanks