This is quite common, and there is usually a reason for it. For example, they may not yet have finalised the equipment.

Also, do you have a connecting flight from AMS? You can't check in until your connecting flight is open too. Sometimes (in my experiences) connecting flights are open before the 30 hours for the benefit of connecting passengers, but more often than not they are not.

If you phone the number in the right-hand corner of the check-in screen they will be able to tell you why check-in isn't yet open. If you're very lucky the response might even be "I can see no reason why we can't open it now, give it five minutes and I'll open it for you".