Sorry if this has been explained elsewhere on FT. What is the possible explanation for the following message received when trying to check in for our international flight: There are two passengers on the PNR, one was checked in with no problem....

We are unable to check in all passengers.

Some passengers are not eligible for online check-in at this time. These passengers will need to proceed to the airport and see a Customer Service Agent in order to receive their boarding pass(es). All other passengers have been checked in and may receive their boarding pass(es) at this time.

Thanks... again I apologize if this message has been explained elsewhere on FT.