What are United Airlines' criteria for issuing a boarding pass to a passenger checking in online for a flight departing the US (to Germany, if it's relevant, with a connection to Croatia)? The passenger in this case has a Croatian passport, and was just told that the airline could not issue a boarding pass by the online check-in system. Why?
To be more precise, I am curious to know whether United will not issue the boarding pass without first confirming the traveler's immigration status in the US. I suppose it might also have to do with public health considerations in this time of COVID-19. But neither of us have much experience flying to Europe on US-owned airlines, so we do not know whether this is a usual or unusual occurrence.