Why the check-in screen can be confusing
You are checking in for a one-way flight to Bangkok. Your passport has scanned successfully, your seat is selected, and the app suddenly stops at a message about return or onward travel. There may be a place to upload a document, a request to contact the airline, or no explanation at all.
It is easy to read every version of this message as the same problem. But the system might be checking destination entry conditions, asking for a travel-document review, or simply refusing to issue a mobile boarding pass until an agent sees your paperwork.
KLM says its online check-in process depends on travel documents being complete and offers document scanning in advance. British Airways also allows passenger information to be entered through Manage My Booking. These examples show why an app can ask for more information even when the flight itself is booked and paid for.
The first task is to read the exact wording on your screen. A request for a passport scan is not an onward-ticket request. A message telling you to see an airport agent is not proof that your flight has been canceled.