First, there may be a typo in the problem statement. It should say either "Let $S(n)$ be '$n^2-n+41$ is prime'" or "Let $S$ be '$n^2-n+41$ is prime for all $n$'". Recall that a proposition is something that can be either true or false. In the first case the truth value of $S(n)$ depends on $n$, and for each concrete $n$, $S(n)$ is a proposition. In the second case the truth value of $S$ does not depend on anything, and $S$ itself is a proposition.Let S (n) be the sentence
n squared - n + 41 is prime for all natural numbers n.
S (41) = 41 squared - 41 + 41 = 41 squared, which clearly is not prime.