I am not able to edit existing LTspice directives; I always need to create new
Not sure if this is the type of directive you're talking about, but the solution is to ignore LTSpice's new and fancy parametric GUI that doesn't seem to actually edit anything, at least not when I try.

When I try to edit a simple .STEP command I'm greeted by this dialogue that often does not work:



If I hit Cancel here, I'm instead greeted with the old edit window, which always works:



You can go directly to the old working edit window by holding down Ctrl while right-clicking on the command you want to edit.