ThroughtheLightx. U need to get at least 95,000 to get S rank but you also have to acheive a certain amount of points more than the opponent. U might not have got S rank because your's and your opponent's scores may have been too close. Go to this site, it shows you how to get the S rank but you might also need to upgrade your perks:

 https://lovenikki.world/guides/stage-walkthroughs/chapter-10/