Hi there. The exotic is not rewarded immediately at the end of the mission. To get it requires that you pick up the rifle item at the end of the activity, wait for the timer to appear, then visit Zavala to get the exotic. If you didn't pick up the item or wait for the timer before leaving, then you will need to make sure you do those steps when completing the mission again. If the quest is gone, then check for it at the Quests Archive next to the Posmaster. If it's not there, then try doing the quest again on a different character, otherwise you will have to wait until tomorrow's weekly reset to do the quest again.