Of 'yapmak'. 'Bunu senin yapman' is relative clause here.
Lazım(It is necessary.)
Ne lazım?(What is necessary?)
Bunu (senin) yapman(That you do this.)
So 'Bunu senin yapman' is the subject in the form of a relative clause and 'lazım' is the predicate despite not being a verb.
For a better understanding you can analyze the relative clause. For example we can turn it into a finite indicative sentence:
Bunu sen yaptın: You did this.
It means the same, and if you were just translating a story or something, that would be fine, but Duolingo is trying to get us to understand "lazım," which is not a verb and doesn't conjugate. "You" is also not in the subject case. It's useful to think of "lazım' as meaning "it is necessary" and the rest of the season as meaning "that you do this" or "for you to do this."