Originally Posted by
avalyn2
Fun fact: I initially registered to the Air Europa website and was not able to enter my FB number, so I tried to buy the tickets as a guest and I was able to do so.
If you mean that you logged into the UX website, then it is not at all surprising that you couldn't add your FB number. UX no longer uses Flying Blue, but started their own frequent flyer programme (FFP) - called Suma - some years ago. Usually, logging in to a website means that the only FFP you can be associated with is the airline’s own FFP.