Hiding Expression columns in a Pivot isn't as straightforward as in a Straight table. There are multiple workarounds to get it to work, but they all depend on what you are trying to accomplish. The example below uses Avinash R's technique to show and hide an expression by selecting its name from a list.
Another solution would be to create an artificial dimension (using ValueList() function) to automatically hide an Expression column without values
To get it to work in your document, please provide more details or upload a sample app, as requested.