Your "then" part of the statement is an aggregate function however in the "Else" part it's string and therefore you are getting an error. Can you send the .twbx file so I can help you?
It's easier to answer these kinds of questions with a packaged workbook as Priyanka suggested. Here are a couple of links that will be useful for attaching a packaged workbook:
When doing so please post the version of Tableau Desktop that you are using it really helps the helpers save time
With that being said, here is a quick idea in formatting negative numbers to display target reached.
Here are a couple of links that are a bit more informative:
I hope this get's you pointed in the right direction.
You have some useful answers already, I just wanted to clearly address your original issue...
The problem you have is that both the TRUE and FALSE outputs from an IF statement should be in the same format.
At the moment your TRUE = INT and FALSE = String.
INT <> STRING hence the error.
Shown here -
I hope that is clear?
The quickest fix -
The easiest fix for the above issue (your issue) is to convert the INT > String.
The problem is that you will no longer be able to sum the column, they will be strings. (you could have the original side by side for calcs if needed)
Here I fix my example, I convert the INT to String using STR()
I haven't tried, but Gerardo's solution may work also.