simple...or expert? This mate in 2 you choose if is!

Pedro,

If you claim to have created a mate-in-two puzzle, that means that you have created a situation where a mate is forced in two. That means that after the first move, any legal move can be followed by checkmate. This means that the first move is generally forcing in someway. Otherwise, it will include the concept of zugzwang (chessopedia if you don't know what that is). To create this type of puzzle, you need to test every legal response to your first move, and see if it can be followed by checkmate. As you can see, 1...cxd6 is a blunder and is followed by checkmate, but nothing about 1.Nxd6 forced black to respond with ...cxd6. There are many moves that black can make that will not lead to checkmate, and so, it is not a mate-in-two puzzle.

Hope that helps!

TonightOnly 

What he said is right, white mates in 2 only if black blunders

Black may be able to get out of it.

Exactly. 

Your “mate-in-two” can easily be avoided by not capturing the knight. 

P.S. Zugzwang basically means that if you are in zugzwang, any move you make will result in a losing position. 

Hope this helps!