You misplaced some parentheses, which confused the compiler. Here's the correct version:
IF(Net_Promoter_Score__c < 7, "Detractor",
IF(Net_Promoter_Score__c > 8, "Promoter", "Passive"))
Generally speaking, if you have a different number of
) in a formula, then you have a mistake somewhere. In your original formula, you had 2
( and 4
), which was incorrect.
If statements are in the form
IF(Condition, TrueValue, FalseValue). When you need to chain them together, they take place of the TrueValue/FalseValue places, like this:
If(Condition, TrueValue, FalseValue),
If(Condition, TrueValue, FalseValue)
Just remember that the left and right parentheses should always match.