Like a novel, a novella will usually have a conflict at the centre of the story. Unlike a novel, in a novella there is usually just one single conflict rather than various things happening.
In Fahrenheit 451, the conflict within the central character is his growing refusal to take part in his job, and his whole society's obsession with book burning.
The conflict or questioning that takes place in your novella should connect all the elements of the story like the spokes on a wheel..
This main conflict should:
Not be straightforward. 40000 words can allow for something to be either delightfully or gratifyingly complex yet allow resolution in that space.
It should be engaging enough to sustain the whole story without any subplots, which might well get in the way or distract from the focus.
The conflict will probably fall into a three-act form - - rising in stakes and expectancy and maybe apprehension over the first two thirds so it can be resolved in the third.
Don't forget that with this conflict, it's important that you somehow attach every aspect of the story to it somehow - that is to say - you should make every aspect of the story relevant to or relate to this conflict somehow.