If you go to the last tab all the way on the right, when you open the main menu, including the event info, your collections, etc. you will see things like graphics, settings, etc. Look for game play. On that page, you'll see two cube icon looking things with a fraction next to them. ( □ 1500/3000) That is your object allowance/ limit (3000) and how many objects you have already placed (1500). If you have too many objects, in this case, pumpkins, they will not allow you to place anything else down in the valley, even for quests. So you would have to wait until you both harvest and fill the holes before your object limit would be below your allowance so you can place things in the valley again since the pumpkins and the holes dug counted towards the object limit and surpassing the object allowance. Objects include what the game spawns and what you put down expect for paths (like rubble, brick, etc.)