The reason is : We may assume the prime ideal with norm $p^n$. Note $(p^n)=(p)^n$, and the prime ideal $I$ with norm $p^n$ divides $(P^n)$, thus $Ip_1...p_m=(p^n)=(p)^n$, thus as $I$ is a prime ideal, we get $I$ divides $(p)$. However, this is impossible since $N((p))=p^2$. Thus $I$ doesn't exist.
But I don't think then conclusion is right, so can you tell me where I went wrong?