Can't uncheck boxes in Pivot Table dropdowns
 

I can't check or uncheck boxes in the Label pull-downs of my pivot table.
For example, in order to exclude the "(blank)" column. I tried starting a
new file with a test pivottable, and it has the same problem. I have also
tried opening a file i created on a different computer which i know
functioned correctly, and it exhibits the same behavior.

Is there a way to fix this?