I notice that when I purchased my one way flight from Granada to Barcelona, I did not get an option to chose or see where my seat would be. I looked for info on the website that might give you that information adn did not find anything helpful. I am to assume that I have to wait til I check in. I assume they have an online check in service. Would that be correct, I chose my seat at check in, maybe the day before flight? Or is it earlier than that?
You have not received a reply in more than a week, possibly because you did not say where you are flying from. That leads to guesswork. Vueling does offer seat selection on its website. The trouble, then, may be with Iberia, not surprising since its merger with British Airways. BA has long been known for its stingy policies regarding advance seat selection. Contact them directly to find out what the rules are.