Error code 51 is Steam's generic way of saying a game failed to start. It appears as “Failed to start game (error code 51)” and gives no detail about what went wrong, so the cause has to be narrowed down by elimination. The usual suspects are corrupt game files, antivirus software blocking the launch, outdated graphics drivers, missing runtime libraries, or another program interfering.
The fix, in order
Restart the PC, then launch the game again. A stuck Steam process or a half-finished update can cause a one-off failure.
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.
Run Steam as administrator. Right-click the Steam shortcut, choose Run as administrator, and try the game. This rules out permission problems with anti-cheat drivers and system files.
Update the graphics driver from the card maker, NVIDIA, AMD, or Intel. If the error began right after a driver update, roll the driver back instead.
Check the antivirus. Add the Steam folder and the game folder to the exclusions, or pause real-time protection briefly to test.
Close overlays and background programs, especially other game launchers, recording tools, and overlays, then try again.
Reinstall the Visual C++ redistributables and update DirectX and the .NET Framework, since a game that cannot load its runtime libraries fails with this code.
Flush the Steam configuration. Close Steam, press the Windows key and R, type steam://flushconfig, press Enter, then reopen Steam and sign in again.
If it still happens
If one game keeps failing after all of these steps, uninstall and reinstall that game. If every game fails, reinstall the Steam client, keeping the steamapps folder so the games are not downloaded again.
Common questions
Is error 51 the same as error 80? No, although the fixes overlap. Both are generic launch failures, but they are different codes with different triggers, so a fix that clears one may not clear the other.
Can a mod cause error 51? Yes. Mods and third-party launchers can block or crash a launch. If the game is modded, remove the mods and test the unmodded game before trying anything else.