You can only get 1 mythic schematic each week, you'll have to wait for it to refresh and then complete the 'Weekly Scion quest' before jumping back into MSK to get another Mythic chest
First step ☝️ Max out all your legendary Survivors with survivor supercharger, make sure they have matching team perks and personalities. Be at least power level 145. These are what you need. You cannot do MSK under powered. You’ll just annoy and get carried. Hard factual truths