Customer claims order not received

Last week, a seller-fulfilled product was marked as delivered to the customer’s house, and there is a delivery photo showing it at the address.

However, the customer claims it was not delivered.

It’s a strange situation, what should be done in this case?

I visited a friend yesterday at an apartment complex, and seeing how Amazon packages were piled up in the mailroom made me realize how easily one could go missing. It immediately made me think of that message that pops up in Seller Central: “Where is my stuff?”