Can I book an international flight w/o passport?

Hi everyone, for context, my parents haven’t been outside the country and I want to bring them to Vietnam next year, so I am planning to wait for an airline seat sale and book our flight. Problem is my parents don’t have passports yet. I am going to ask them to get passport of course but just in case, the seat sale comes first, can i book them a flight without passport? Is that possible? Thank you.