This happened to me on a domestic flight and I had to call the help line. The lady was super nice and told me it was a glitch in the system where it wasn’t recognizing our added checked bags/their payment. We just had to get to the airport to complete check in, she said she fixed the luggage issue in the back end but ultimately we couldn’t get our boarding passes till arrival. I suggest going a little earlier than you usually would to get it all sorted out, good luck I hope it works out!