Starting navigation from the Explore iOS app on Enduro 3

When I select a Route in Garmin Explore iOS and select "Navigate", the Explore app just shows a pop-up that says "Connect a Device to Navigate" and then shows a spinner titled "Searching for Devices". Nothing else happens.

I have an Enduro 3 watch and it is paired with the Explore app (it is shown in the list of devices) and Bluetooth is on. My expectation is that the following happens as described in the Explore app manual:

"The selected course, route, or track will be shared with the connected Garmin device(s) and navigation will begin in BOTH THE APP AND ON CONNECTED DEVICES(s) once GPS has been acquired."

However, nothing ever happens on my Enduro watch. What is going on?