One of the gifts I got from the Socking Stuffing event was a Draeyl Father Winter costume. It was sent to the Player Trades tab, and I keep trying to accept it, but it says that my inventory is already full. I went and checked, and even bought a second tab, but it still will not let me accept the costume. This also happens when I try to open mission bags, it says that I need to clear up inventory space. I do not know if this is a glitch, or something that is supposed to happen, but it has been happening for a while, and I do not know what to do.

was your equipment inventory full? that is usually the problem.

Mememachine is likely right about what your issue is. Costumes go to the equipment area of the inventory, and often the mission satchels have equipment in them as well. If the equipment area is full, you won't be able to open the satchels containing equipment or accept equipment/costumes from trades. To fix this, switch from inventory to equipment and clear some space, or buy a new tab for the equipment inventory.

Thank you, that was the problem!