Indeed, you get the second half of the warning only in the case of surface source models. In your case, the volume grid cannot be fixed with this menu "Force inside skull".
Can you give me some suggestions on this? Did I choose the wrong surface file to force inside skull?
Sorry, I sent you on wrong tracks with this menu "force inside skull". If you use a volume head volume and a template grid projected in the subject space, there is no easy way to fix the position of the grid points.
Keep working this way, just make sure that you don't see abnormal values at close to the skull.