 Countable rank-initial segment cannot model ZFC

If $\alpha$ is a countable ordinal and $V_{\alpha }\models \mathsf{ZFC}$, then $\alpha$ must be a limit ordinal. Choose a countable cofinal sequence $({\alpha }_n)$ in $\alpha$. The internal axiom of choice gives, for every $n$, a bijection between $V_{{\alpha }_n}$ and some ordinal below $\alpha$; that ordinal is externally countable, so every $V_{{\alpha }_n}$ is countable. Hence $V_{\alpha }={\bigcup }_n{V}_{{\alpha }_n}$ is countable. But $V_{\alpha }$ contains the full power set $\mathcal{P}(\omega )$, which is uncountable by Cantor theorem, a contradiction.