You can't set the trace widths of a diff. pair to be less than the design rules. So somewhere in your settings you have a rule for a 6 mil trace width for those nets, and that is what needs to be fixed. I recommend creating a class for the nets in each of the different values of diff. pairs (90 ohms, 100 ohms, etc.) just to set the trace width to the right value. Then you should be able to set the width and spacing in the dialog box you show.