If the Xbox sync button is not working, replace the controller batteries first and see. This article lists some fixes that will help you.

You can also resyncing your Xbox controller can resolve this issue. Look on the top of the Xbox Wireless Controller to find a sync button next to the USB-C input port.

To wirelessly connect your Xbox Series X or S controller to the console, press and hold the Xbox button on the controller until it flashes. Then, press and release the Sync button on the console ...

Connecting your Xbox controller just takes a few simple steps to connect to your Xbox.

Turn on your Xbox Series controller by holding the Guide button in the center. Now press the the small sync button on the top of the controller until the Guide button begins flashing.