Error code 80 appears when Steam tries to launch a game and the launch fails before the game window opens. It is a generic launch error, so the message does not name the cause, but in practice it comes down to corrupt or missing game files, a Steam folder set to read-only, an outdated graphics driver, or another program interfering with the launch.
The fix, in order
Restart the PC, then try launching the game again. A stuck game process from an earlier session can block a new launch.
Verify the integrity of game files. In the Library, right-click the game, open Properties, then Installed Files, and click Verify integrity of game files. Steam replaces anything corrupt or missing.
Check the Steam folder is not read-only. Right-click the Steam installation folder, open Properties, and clear the Read-only checkbox, then apply the change to all subfolders and files.
Update the graphics driver. Download the latest driver from the graphics card maker, NVIDIA, AMD, or Intel, and choose a clean install if the installer offers one.
Close conflicting software. Overlays, recording tools, and RGB utilities can block a launch, so close them and try again. A clean boot of Windows isolates the culprit if the error only happens with other programs running.
Run Steam as administrator. Right-click the Steam shortcut and choose Run as administrator, then try the game again.
If it still happens
If the error follows one game only, uninstall and reinstall that game. If every game fails with code 80, reinstall the Steam client itself, keeping the steamapps folder so the games do not need to be downloaded again.
Common questions
Does error code 80 mean my game is broken? No. It means Steam could not complete the launch, and the usual causes are file or permission problems on the PC, not a broken game. Verifying the game files fixes it in most cases.
Why does error 80 appear after a driver update? A new graphics driver can clash with a game or with Steam itself. If the error started right after a driver update, install the previous driver version and test again.