WP Travel Pro plugin provides the feature of allowing admin (who manages trip) to either add an inquiry button or display the Sold Out message in 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 in the individual trip setting page.
So, if you are getting trouble regarding 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 Out Of Stock option, please refer the link of our official documentation
If you have any confusion then please post them in our official support forum.