Customer services say for security 20 passages per flight must check in this way
Not sure about that, but if that's what they say then.....
What I do know is that they don't allow the whole aeroplane to be checked in online, not sure why but there will be a good reason, so I can only assume that you have missed out on the online check in by leaving it too late. If the check in process is as well organised at STN as it is at the other bases then you won't have any issues, you'll be done in a few mins and will have had your first taste of the Jet2 experience which, all being well, will make you change your mind about ever travelling with Valdermort Airlines (FR) again.