On Jane Friedman site:

Story structure promotes the concept that every scene of your story should serve a larger purpose—e.g., the inciting incident sparks the problem, and the climax eventually brings everything to a head. Your plot should act like a Rube Goldberg machine: an intricate design of turning points that need to be hit correctly. If a single domino tile isn’t in alignment with the hammer that will push the bowling ball onto the track, the machine fails.

Maintaining forward momentum is vital. No piece can be misplaced.

You’ve undoubtedly heard of countless theories about story structure, all with different names: Three Act Structure, Save The Cat, and Kishotenketsu, to name a few. No matter the name, all plot structures prescribe milestones of change your characters need to hit to fit into your genre.

Middles are especially hard because, even when you study structure, the beginning and endings are more regimented and obvious. Unfortunately, that can mean the larger—and just as important—middle is left sagging.

So what do you need to succeed?