Depop marked as delivered in mailbox

For reference, I live in an apartment and bought something off Depop which I’ve never had a problem wit up until now. The tracking for the item was stuck in the same spot for damn near 2 weeks while the seller said he’ll figure out what’s going on and then randomly the next day it’s marked delivered. I went to check and there was nothing there and Depop is refusing to give me a refund because it’s marked delivered.