Is your inventory full? You would still be able to take stackable items in that case, but nothing that would use a new slot (keys, equipment, books, etc).
Just to the right of the inventory tabs is a fraction showing the current item count (excluding quest items) and the maximum size (100 plus any bonus from the Encumbrance skill, though if you are a little selective in what you keep, use any skillbooks that you acquire, etc, you should easily be able to stay within the default limit until you get access to the battle tower chest and can offload alchemy and enchantment ingredients and anything you're not going to use, but don't want to get rid of [equipment set pieces you might want to try with the full set, etc]).
