Infinite Phase Margin
Your system \$\frac{1}{s^2 - 9}\$ has gain less than 0 dB at all frequencies. The bode plot you have plotted shows that clearly. Since the gain doesn't cross 0 dB anywhere, the phase margin is not well defined.
Apart from the above, this system is open loop unstable. The open loop system sys has a pole at \$s = +3\$. For systems which are open loop unstable like this system, use the full Nyquist criterion (counting the encirclements) to determine stability.