Hi
@NG,
If X6 is a dummy factor and a constant value, is there a particular reason you keep it in your analysis ?
It may be easier to simply consider X1 to X5 to add to 100% (even if it is in fact 50% of your total formulation), and drop the X6 factor. Since Mixture design is all about ratios between your mixture factors, having a total at 1 or 0,5 won't change the ratios (and solution(s) found), only the absolute values/quantities of your factors.
Or else, you can try to include X6 as a constant factor (not a mixture one) with a value of 0,5 and apply the constraint X1+X2+X3+X4+X5+X6 = 100% (or 1).
The first or second option should bring a solution to your case.
I hope it will help you,
Victor GUILLER
L'Oréal Data & Analytics
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)