If you have tried every single step above and the error persists, the issue may be a deeper OS corruption or a hardware conflict. In that final scenario, contact Square Enix support or post on the Steam Community Hub with a detailed list of what you’ve tried. But for the vast majority, one of these solutions will be the key that unlocks the mayhem. Happy grappling.
If the steam_api64.dll file is missing or corrupted, the game cannot start.