One of two things I think are happening with referrals codes with Model 3 in Europe:
They're not applicable yet just like they weren't until August in North America or
Because the current program expires on Monday December 10th there's a chance the whole program will come to an end
If #2 indeed happens then it makes sense to not offer any referrals for fear of upsetting too many people who haven't ordered yet. If the referral program is renewed on Monday with adjustments and new prizes then we'll just have to wait and see if they allow it in the future.