The Rank 71 reward (Weapon Scrounger) is a drop rate increase; these are generally automatically applied to your account as soon as you reach the requiredSeason Pass level (no need to claim the reward). Below your Season Pass Rank track there should be a series of unlocked bonuses currently applied to your account. The Scrounger Bonus set should be the second from the right and (assuming you have not yet reach Rank 81) should show as two/thirds full with both Armor Scrounger and Weapon Scrounger highlighted as being unlocked). If it is not displaying as such you should try clearing your cache as this occasionally helps to remedy random issues. (΄◉◞౪◟◉｀)