If you've booked a basic economy or light fare ticket, it is the airline's most restrictive non-refundable fare. The airline will not allow you to make any changes or cancellations to the itinerary and they do not make exceptions.
If you'd like to avoid basic fares, Hopper allows you to filter them out of your search results to ensure you end up with the comfy seats you are looking for!
Advanced seat selection is not available for basic economy bookings and families will often not be able to sit together. Some airlines may allow you to select your seats in advance for a fee.
Depending on your itinerary, a carry-on bag may or may not be allowed. In the case that a carry-on is not permitted, you will be required to check your bag and pay the airline's checked bag fee.