When is this trip?
If it is tomorrow and it says that there are no seats available, than it is possible that indeed the train is full. If you have a 1st class ticket and end up not having a seat you can ask for a partial refudn.
If it is in a few days or weeks, and there are no seats offered when you try to reserver it is probably just not possible to reserve seats for that trains. If it is months in the future then probably the railways don't even know which trainset they plan on using on that train, and thus have not loaded any seat inventory yet.
Trains in Europe do not sell out days in advanve. If they would, they would bbe useless. Most of the people you will be sharing that train with will have bought their ticket on the day of travel itself, and will not have reservations.