How do I fix Apple TV 4K fast blinking light???

Apple TV would have a flashing status light during startup, during updates, and continuously when failing to start up.

If it fails to start up, first try disconnecting from power for a minute or two, then try again.

If the issue persists then it needs to have its software restored. Restore your Apple TV - Apple Support. For the 4K model, users can’t do this themselves through a computer. Instead contact Apple Support to have it restored for you.