(App already Running)
This error can happen when you try to launch the game multiple times, or if a previous run of a game did not properly close.
The easiest way to resolve this is to restart steam, and if it persists after doing this restart your computer.
Failed to start game (executable missing)
Your antivirus software has likely eaten your game’s executable, falsely mistaking it for tasty malware.
Add your game folder to the exceptions/permissions for your firewall software, and then revalidate your steam file by right clicking the game in your stime library, and selecting > Properties > Local Files > Verify Integrity of Game Cache to redownload any quarantined files.
You can find a collection of antivirus software with configuration steps HERE.
For further assistance with the Steam Client, we recommend contacting Valve’s support team HERE.