Verify that you are using a compatible controller.

If you are using an unlicensed accessory, replacing it with a licensed controller may resolve the issue.

Check the controller’s cord for any signs of physical damage, such as a frayed cable, nicks, kinks, etc. If any damage is present, the controller will need to be replaced.

Power off the system and unplug the controller. Plug the controller into port number 1 on the front of Nintendo Classic Mini: NES and try starting a game.

If possible, try using another compatible controller. Ensure it is plugged into port number 1.

If another controller works correctly with the system, then the original controller will need to be replaced.

If the issue persists even after using another compatible controller, or if another compatible controller is unavailable, then the console and the original controller will need to come in for repair. Please see here for more information.