If your story revolves around a conflict between two characters, this better be a strong conflict that evolves and has reason to persist.
Maybe you're writing an enemies-to-lover story. Maybe your protagonist has to reconcile with their parents. Maybe your character has to figure out how to survive the worst boss in the world.
Whatever the conflict, it has to be durable. People can't be mad at each other for petty things that should blow over in a week or that can be resolved with one conversation and explanation.
Your conflict has to be worthy of the length of your story.
Make sure you find the twists and turns that make your conflict keep your readers looking for a resolution throughout the story.