If Steam displays the "preparing to launch the game" message but the game does not want to launch, try logging out of Steam and sign back in.

If it does not help, you can try searching the Net for a file called xinput1_3.dll and place it into Dying Light's main folder, which is located at C:\Program Files (x86)\Steam\SteamApps\common\Dying Light by default.

Additionally, see:

128 MB / Missing Executable

Official PC system requirements for Dying Light