No prior professional experience is necessary, anyone from any industry will benefit from this course! And even if you aren’t managing clients now, these skills are useful and transferrable to any professional interaction.
That said, we can separate students and potential students into two types:
Those who have been burned; and those who are too young to have been burned.
Let me say that differently. In everyone’s career,  you will face one point where clients or bosses treat you terribly. Often taking advantage of you. Often in very serious ways.
The younger students are the ones who want to master the art of client management before that burn turns to them in a serious way.
The mid-career students–I don’t want to say “older” because that would put me into that category!–are the ones who have experienced it and want to up their client-management game to the next level.