Steam Play allows you to purchase your games once and play anywhere. Whether you have puchased your Steam Play enabled game on a Mac or PC (both Windows and Linux), you will be able to play on the other platform free of charge.
Look for the Steam Play icon or when shopping on Steam.
A common reason for this error is that the game simply has not yet been released on the platform that you are attempting to play on. Additionally, please ensure there is not both a PC and a Mac listing for the title in the Library list.
For a complete list of available Mac and Linux titles on Steam, please click the following: