It may be that technical prereqs are missing. It could also be that you're missing a broader sense of "mathematical maturity", or that you're struggling because Stuart's work is simply hard to understand. That said, useful prereq areas (in which you could also gain overall mathematical maturity) would include:
It's probably overkill to go deep into these topics. Usually, what you need is in the first chapter.
I've looked at a few of Stuart Armstrong's posts that he put up related to his research agenda (though only ones before he posted the full agenda), and felt like I was missing some prereqs. My background is in philosophy. What subjects or particular resources should I study to be able to read his work?