Some more advice from the Big Fish forum:
Well, if it's still *daytime* you won't be able to get the mandrake, cart or not. So instead, give up, go back home and talk to Nostradamus. Or check a few rooms again. See if there was anything you missed picking up. You will be in the house when night falls and then you'll have to make your way back to the gallows in the dark.
Not knowing what you have/have not done, it's hard to get more specific than that.
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