score:4
I don't think it will be possible to choose a window seat. Quite likely they are actually all taken or, at least, assigned. You could (and should) keep doing online checkins in case people change their minds; a seat might become available.
I suspect your lack of a seat reservation was caused by booking through a third-party site. It might make sense to book your tickets directly with the airline. Alternatively, when you use a third-party site you should get a PNR (reservation) code that you can use to go to the airline's site to reserve a seat later.