I am reading a book that is describing a group of people in the middle ages. It says "[they] had managerial and financial jobs, jobs of high complexity, and were neither farmers nor craftsmen". I am not trying to be PC but I do not understand the complexity of managerial and financial jobs. I am hopeless as an artisan, measure things wrong and can get irritated when facing problem-solving puzzles when home-fixing. I have no experience in finance (I have mostly worked in healthcare and science) so I am genuine when I ask: what is the "high complexity" of financial jobs in the middle ages? I see it as writing down who owes who, maybe calculating percentages... not much more.
"Writing down who owes who, maybe calculating percentages" may seem trivial to you, but that is because you have had the benefit of several years of modern education. In middle age Europe, there were essentially no schools. Education was reserved almost exclusively for those wealthy enough to employ a teacher (meaning aristocrats) and for clergy who needed to be able to read (and sometimes write) scripture. Thus, it's not clear where you propose that people should have learnt to write and calculate in the way necessary to do these jobs. Some estimates put the literacy levels at the time as low as 6%, so we are really talking about the intellectual elite of society at the time.
As an aside, modern-day financial jobs can be extremely complex. Some of the top jobs at modern financial institutions are occupied by so-called "quants"—PhD-holding mathematicians, economists, and scientists whose job is to build complicated mathematical models of markets and asset prices.