You should use != to indicate not equal to. So
[MyFirstModule.Patient_Treatment/MyFirstModule.Treatment[Outcome != 'Positive']]
Or when not comparing multiple values you can write the short version
[MyFirstModule.Patient_Treatment/MyFirstModule.Treatment/Outcome != 'Positive']
Also, it's not a string even though you type it as a string, you can only enter valid enum values there.
I have requirement for [Length=$IteratorStagingTable/length] in xpath constraint.
Where Language is attribute of Asset Table in DM and length is attribute of IteratorStagingTable ,when i do above syntax in xpath for length string type its work fine,but for type length as enumeration its showing :Incompatible expression types:Enumeration