As per the below link, Project Purchase Orders are not supported by the Landed Cost module.

https://learn.microsoft.com/en-us/dynamics365/supply-chain/landed-cost/manage-voyages

As it stands the system will still allow you to add a Project PO to a Landed Cost voyage but it will create incorrect financial postings.

The Landed Cost module has become a central place for customers to manager their incoming inventory receipts and the compatibility with the Project module will allow them to have all incoming deliveries managed in one place.