Yes, I’m referring to the property of the column but it seems that you are applying the formula into a format rule. I think you can use the SUBSTITUTE in the column properties because you are changing a data.
What you can do here in format rules is to change the formula into [Antigen] = “Positive”, and you will add an icon, select a color, change the property of the font such as bold, italic, strikethrough, etc.
Then just create a virtual column and apply this formula SUBSTITUTE([Antigen], “+”, “Positive”) because you are trying to change the data. You cannot change the data in format rule.