Writers are often told by teachers & screenwriting gurus to refrain from using flashbacks due to fears of misuse. But A LOT of our favorite stories and films in history use them A LOT!
https://www.scriptreaderpro.com/how-to-write-a-flashback-in-a-script/
Flashbacks in and of themselves are fine but pay attention to what Jack said. Do not use them as crutches or the 'easy' way. like all tools, learn how to use 'em. Learn when, where and how to use them properly and understand when they are inappropriate. Don't be scared of 'em.
One aspect absent in the article is that a flashback must be shown from the character’s POV and/or show the character in the flashback. Even released big-budget flicks contain flashbacks in which the flashing character was not actually there.
There are different types of flashback Eg Audience sees something that happened in characters past so learns reason why they behave the way they do now. Or some flashback disturbance that is an action that still has yet to be understood. Eg A man keeps banging on a car in the rain but we yet don’t know why. Also there are dream or memory flashes. @Doug is right, never be afraid of flashback but understand its place. Some writers try to be funky with flashback but reader feels their brain is a tennis ball going back and forth, here there, back then now. Flashback must serve story to higher end, intrigue, reveal or surprise. It is not a decorative tool to make page look a certain way or to be used simply to make story non linear. Stories are organic, breathing entities and do not have a one template fits all.
I studied under Linda Aronson, queen of tandem, flashback and non linear, this statement in photo looks like some Einstein formula. There's a great book called 21st Century Screenplay that is a must read if you cannot do her courses.