WP Travel Pro plugin provides the feature of allowing the admin (who manages the trip) to either add an enquiry button or display the Sold Out message on a single trip page if the trip has been booked up to the maximum number of pax that you have defined.
This feature can be achieved through the inventory option which is available on the individual trip setting page.
So, if you are getting trouble regarding the Out Of Stock option then please make sure that the inventory option has been enabled or not by going to Admin Panel > Trips > Individual trip(edited) > Inventory Options
For detail assistance regarding the “Out Of Stock” option, please refer to the link to our official documentation
Further, if you have any queries, please submit them to our Contact page.