Custom Button - Location Choice is not changeable in creation

Custom Button - Location Choice is not changeable in creation

I created a custom button for the Tasks module that calls a function to streamline mass updates of closing tasks as "complete".

I want to place it in either the utility menu or the mass action menu, but when I create the button the "select page" field is grayed out and no choice appears except for "In Record".

This is not my first button creation - why is this happening? 

    Nederlandse Hulpbronnen