  Let $V\subseteq W$ be two operator spaces. Arveson-Wittstock-Hahn-Banach
theorem asserts that every completely contractive map $\varphi:V\to
\mathcal{B}(H)$ has a completely contractive extension $\tilde{\varphi}:W\to
\mathcal{B}(H)$, where $\mathcal{B}(H)$ denotes the space of all bounded
operators from a Hilbert space $H$ to itself. In this paper, we show that this
is not in general true for $p$-operator spaces, that is, we show that there are
$p$-operator spaces $V\subseteq W$, an $SQ_p$ space $E$, and a $p$-completely
contractive map $\varphi:V\to \mathcal{B}(E)$ such that $\varphi$ does not
extend to a $p$-completely contractive map on $W$. Restricting $E$ to $L_p$
spaces, we also consider a condition on $W$ under which every completely
contractive map $\varphi:V\to \mathcal{B}(L_p(\mu))$ has a completely
contractive extension $\tilde{\varphi}:W\to \mathcal{B}(L_p(\mu))$.
