They might already be reserved by villagers, Market Tenders and Transporters on their way to the Granary to pick them up. Try clicking on your berries in the resource window to stockpile them (the quantity will turn yellow when stockpiling). This will prevent any villager from taking berries from your granaries so you can build up enough for quests. Make sure to stop stockpiling(clicking berries in the resource window again) when you're done so they don't starve though :)
edit: also, when you have enough stored the red X next to the request will turn to a green checkmark that you can click to complete the quest.
Post edited July 15, 2020 by zornochio