The carrier's tracking indicates that the package has been delivered to the recipient, but your client claims not to have received it:
The carrier's tracking is authoritative: if the package is marked "delivered" there is no possible recourse.
Often the package will have been left with a neighbor or the client does not have a standardized mailbox: in this case, you should advise your client to go to the nearest Post Office (if the carrier is the Post Office), with their tracking number and an ID to retrieve their order.
If the mailbox is not standardized or the package cannot be deposited there (because it contains too many products), it also happens that the mailman/delivery person leaves it at the post office.
If despite everything the client tells you they have not been able to retrieve their package, the decision is up to you: you can inform them that once marked "delivered," you are no longer able to send them a new order or you can decide to place a manual order at your expense.