How hard is it to become a CTO with no degree?

Typically C-suite at 50+ person companies, who aren't founders, have long, 10+ years careers and usually much longer, usually at a few different companies, and they have seen some things along the way! Your degree stops mattering after job 1, so it's largely irrelevant for becoming a CTO. Being a CTO is way more than just coding and the management/leadership and strategy side is something I've seen in graduates of the most prestigious schools, like Stanford and MIT, so that piece might be something more about you as a person, that these schools tend to attract, that you need to have regardless of the degree or experience.