Yesterday I talked about how we as writers make promises to the reader. Whether in genre or through the first chapter or even the first act, you are telling the reader what to expect from your story. If you don't make good on those expectations, you might have an unhappy reader on your hands.
But does that mean you can never break a promise? Never throw in a twist or have a character ultimately fail? No, it doesn't.
You can break a promise. Maybe you want your reader to feel a certain kind of upset, a betrayal, that comes with throwing off their expectations. For example, maybe it's a character that is supposed to be good no matter what, but they go bad or have always been bad. That registers emotion, and it can be a "good" emotion in propelling a story forward.
Another reason not to meet an expectation is to add tension to the story. Writers actually do this frequently! Does the MC achieve their goal on the first try? My bet is no. Every time your MC fails, it makes the reader more worried that your MC won't succeed at all. They start to wonder how your MC is ever going to get out of this mess, because surely they must get out otherwise it would just suck. That worry makes the eventual success much sweeter.
You have to be careful here, though. You pull the reader's leg one too many times and suspense will turn into frustration.
And finally, what if you break the biggest promise in your book? What if your MC fails in achieving their goals? People (and I'm talking mostly Americans here because it does vary culture to culture) generally don't like to read a story where the "hero" loses. It goes against the general assumption that stories are about someone who, even if it takes a long time, ultimately succeeds in some form.
This is the hardest promise to break, but it can be done. If you want an absolutely incredible example, read L.K. Madigan's Flash Burnout. The key to breaking this expectation is replacing it with something else—likely it's character growth and/or restitution.
If an MC learns a very big lesson or has some kind of revelation, then a reader is usually able to accept the failure. Why? Because in its own way that's a success. Maybe not the success promised, but something nonetheless.
If the MC makes restitution for their failure, then readers are more likely to forgive the MC. They can accept that the MC recognizes the failure and is trying to fix it, even if it's impossible. Readers can respect that, though it's not the way they hoped it would happen.
In other words, if you break the biggest of expectations, there must be some kind of healing to follow. And it needs to be enough that the reader is left satisfied. Maybe not happy, but satisfied that the MC came out of the story with something.