Related to raise to power in equation

Question

I am trying to have raise to power in the following equation, but it gives me the error: Missing \right. Inserted Extra \right.

Here is the latex code for the equation:

\begin{multline}
    P_{out} = \left[1-\exp\biggl(-\frac{\phi}{\sigma^2_{f_m}\bar{\gamma}}\biggr) + \exp\biggl(-\frac{\phi}{\sigma^2_{f_m}\bar{\gamma}}\biggr)\cdot \\ \frac{\exp{(\delta)}}{\sqrt{(\alpha_m^{\ast})^2\sigma_{f_m}^{2}\sigma_{g_m}^{2}}}\cdot 
    \sum_{k=0}^\infty W_{-k-\frac{1}{2},0}(2\delta)\cdot \sigma_h\cdot \gamma\biggl(k+1, \frac{\gamma_{th}}{\frac{A}{\sigma^2_h}}\biggr)\right]^{M}
    \label{final}
\end{multline}

Note: I am using double column, IEEE format.

Any help in this regard will be highly appreciated.