Upvote:4
Low-cost airlines (Ryanair, Vueling, Wizzair, et al, are well-known for being fussy about check-in, sometimes requiring additional steps or even the presentation of documents that don't seem to be required.
This behavior may be a function of their low-cost-ness, with staff being overly careful to avoid putting the airline in a situation where the airline must return the passenger to the departing airport because of a perceived or even possible refusal of entry at the arrival airport.
Whatever the reason, if you want to fly on those airlines (and take advantage of their low fares) you'll have to do what they require. Here, that's an in-person check-in.