No, you will not be able to choose but you may get one.
Airlines block off some seats from online booking in order to accommodate travelers seating together. So there are most likely Economy seats available, even though you cannot see or select them. There is a possibility of overbooking, so in that case you will be given a seat of another type of possible of a different class, although this is increasingly rare.
The last possibility is that you may be given a seat in another flight! This will usually come with some form of compensation, not necessarily monetary, since it is the airline that chose to overbook. What you get depends on legislation and airline policy but Europe tends to be more generous with this type of compensation.