If you were not assigned a seat, this means either the airline is not permitting preassigned seating at this time or they may only assign seating at Check In. Please note, once you have paid for your ticket, you do have a confirmed seat regardless of whether or not you have a preassigned seat number.
Keep in mind that pre-assigned seating is different on all airlines. Some require a fee for preassigned seating; others will close the ability for pre-assignment if it is too far out, or too close in. If preassigned seating is not available at this time, be sure to check-in early! This is usually the time when available seats held for other purposes get opened up.