on Fiction University:

The act three plan kicks off the end of the novel and promises readers an ending to remember.

The act three plan is one of those moments that make it easy to tell if your ending is going to work or not. If this plan is strong (even if it won’t work), and clear about what has to be done, odds are the ending and climax will be equally strong and clear. If this is fuzzy, odds are you’re going to struggle with your ending.

This isn’t an absolute, of course, just something I’ve noticed over the years. This is the moment where the protagonist decides what to do to resolve the conflict of the novel and puts a plan into action. If you aren’t sure what that plan is or what has to be done to resolve the conflict, that’s a big red flag that you aren’t clear about what your ending is. It could also suggest you aren’t sure what the core conflict of the novel is.

“I don’t know how to end it” is a common issue with novels that don’t have a clear problem to solve, and the author doesn’t know what it’s truly about. Once they figure this out, they figure out how to end the novel.