It’s difficult to assign fault knowing nothing about the booking, terms and conditions, etc. - and you’re asking us to make a guess. Which property? What occupancy levels? If it was very recent, “semana Santa” and Spring break for many leading up to Easter probably mean occupancy was very high and there was little flexibility. (Nonetheless, rather than two doubles, you got a double sleeper sofa and a king bed - that might have meant a larger room, and certainly offered separate beds.)

If I were you I’d take up with the booking agency. If they “promised” two doubles (how was that expressed, exactly? Do you have that in writing?) they should have disclosed the property might choose different accommodations for their clients.