The expression is:
$=\sum_{i=1}^{M}[(\frac{1}{L})^N\frac{1}{M+1}] $
The expression is:
$=\sum_{i=1}^{M}[(\frac{1}{L})^N\frac{1}{M+1}] $
If $P(X|L=0)$ is assumed to be $0$, you can remove $i=0$ case from the summation. With some cleaning, we have
$$\sum_{i=1}^{M}\left[\prod_{i=j}^N P(X_j|L=i)\right]P(L=i)=\frac{1}{M+1}\sum_{i=1}^{M}\frac{1}{i^N}$$
This is a p-series and doesn't have a closed form formula.