How to prove that the Permutation cipher is not CPA secure?
If $a_i = c$ for all $i$, then clearly $a_{\pi(i)} = c$ for all $i$ as well, regardless of the permutation $\pi$.

Thus, you can choose your two messages $a$ and $b$ such that, say, $a_i = 0$ and $b_i = 1$ for all $i$.  This will then also be true for their encryptions under any permutation cipher, allowing you to trivially distinguish the encrypted messages with 100% certainty.