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It means the item you will get from the quest as a reward will make that item to excess quantity of 30 in your inventory. So you must look at the quest rewards at the menu, find the same item name on your inventory, and reduces the quantity so it will become 30 or lower.
For example if you have 27 potions and you will get 5 potions from the quest, you should at least sell 2 potions from your inventory.