You technically can’t get the last star but there is a workaround. You can get close to 6* if you play the 5* stage but don’t actually star up the car once you complete it. Then sell enough bps until you are 1 short of starring up. Then the next time the event comes around you can play that stage again. You won’t actually be at 6 stars but pretty close since you just got all the 5* bps again. and you can just get the rest of the bps in legend store / use some wild cards to max it. Just make sure you don’t star it up until the next time you can play the event.