You can't carry any more of those items

Mouse over the item and check if the tooltip says it is Unique.

You can only have one copy of a unique item at a time. If you already own a copy of this item, the game will return the error "You can't carry any more of those items" whenever you interact with a second copy of the same item.

To resolve this issue, find your original unique item in your inventory or bank, and sell or delete it.