No. To make Chicken Marsala you have to use Marsala. Without it, it's like making a peanut butter and jelly sandwich with marshmallow fluff and not jelly. It might taste good, but it really isn't a PB and J.
Marsala tastes a lot different than merlot, is sweeter and has a different consistency as well.
That said, there are lots of other things you could make with chicken and merlot -- like coq au vin, for example. Or you could follow the directions for chicken marsala and use the merlot, but it won't really taste anything like chicken marsala. Might be good though!