Unfortunately there is an issue with the Xbox Controller USB driver that OneCast is using, that prevents the X button from working correctly with newer Xbox One wireless controllers connected via USB. Please note that this only affects newer controllers bundled with Xbox One S/Xbox One X (controller model 1708). Older controllers are not affected.
You can easily resolve this issue by either connecting the controller to your Mac via Bluetooth rather than USB, or connecting the controller directly to your Xbox console. Please see here for details of how to connect the controller via Bluetooth.
We are currently working on a fix and hope to have it resolved soon so that you may use these controllers via USB connection.