Apple Watch 4 notify when left behind greyed out

Article not useful at all. No where does it tell how to set up watch when the option to turn it on is grayed out.

It would have been more helpful to point out if the Series 4 and not be set for left behind and which Series could be.

See this;

Set up and use Notify When Left Behind in the Find My app

It's greyed out because this feature hasn't been activated yet on the watch.