If you are trying to launch a game or a Valve-related application on Windows and encounter the error message, you are not alone.
Work your way down this list until your game launches successfully. 1. Verify Integrity of Game Files
typically occurs when the Steam client or a specific game's executable is unable to find a required function within a steam_api.dll steam_api64.dll
This one action resolves the error in the vast majority of cases.