It sounds like you’ve run into the Inventory Bug. It’s been reported by a number of people, though it seems to effect different items for different people. (I have over 20 items that I can’t smuggle in with briefcases or Agency Pickups.)
It’s on their list of known issues, but you may want to post your details in the Bug Report thread or submit a ticket with Player Support.
(And you unlocked that one by completing the Sniper Assassin challenge on 17 maps.)