I am unable to edit these two pages, Terry Bogard/Appearances and Sans, with the classic visual editor due to them having "complex code" despite not have any code that is different from other pages on the Wiki. How do I fix this?
you can't. if editor says that page is too complex, then the page is too complex. if you think that editor have make a mistake, you can use special:contact to report the problem. but me personally never did see the case where the editor would be wrong.