My living room is small and there is a large cold air duct in the way. Can I put a piece of furniture on top of it?

Maybe, as long as it(the furniture) doesn`t restrict the flow of suction air that is being pulled in
Like a taller type table or something like that

"large cold air duct" Is this supply or return? If it is a return and large, you will block air distribution in the house. If it is a supply, I'd worry about hot dry air and cold moist air blowing on the furniture, particularly if it is wood. BNest to not block any register. You can get diverters that will direct the air flow but you need some xtra space.