On my Titan character (level 15), the postmaster in the Farm has a grayed-out Letter Fragment. It won't let me claim the Letter Fragment due to "not enough space". However, my Titan has plenty of free space available. Therefore, I don't understand why I can't claim this Letter Fragment?
Alright, i had the same thing happen. it means you have a similiar quest item in your inventory, all you need to do is turn in that one to cayde-6 and you'll be able to pick it up