my watch says to set up blood Oxygen on my apple watch on my phone. It shows open on my phone but will not work on my watch. What do I need to do. Brand new watch.

New series 8 received as replacement of malfunction series 8. The oxygen sensor worked on the phone but is not working on this one. I have checked and have what i think is all the widgets checked. It simply states "to set up blood oxygen, open the apple watch app on your iphone. I have done this and it still does not work