Got it,
I got no experience in StudioX but could you please check if you can right click the variable and see if edit button appears?
Also, can you make sure that you can edit variables in variable panel?
Last thing would be working with only one instance of StudioX at the time.
I have experienced the problem of missing edit buttons in UiPath Studio when I had more than 1 instance opened at the time, this might cause a problem as well.