I am trying to change(Substitute) a value to be shown

[Tiger1]

I am trying to change the value to be seen to my user when a certain value is given.

When “+” shows up in my raw data I want “Positive” to be shown in my app. I cannot seem to make this happen. Here is my current expression:

SUBSTITUTE([Antigen], “+”, “Positive”)

The error it gives:
The filter condition ‘=SUBSTITUTE([Antigen], “+”, “Positive”)’ of format rule ‘Positive Color Code’ must return true or false.