Unable to use purchased passenger stands

Airport is Bari.

I had 2 small stands when starting the airport, and purchased 2 more small stands. However once I have 3 planes assigned to stands, I am told “no arrivals available” and cannot use the extra stand.

You need to get more destinations and arrivals, this isn’t a bug mate

Exactly go to connections tab and purchase / unlock destinations (with “golden keys” or in-game money).