Did you press the pairing button on the Airpods case? before searching it via the add sensor settings on the watch? mine connects successfully, it only disconnects from the watch and automatically connect to my iPhone when i enable the Bluetooth on my iPhone.
If its connected to more than one headset, try to turn off the other headset connected(Bedroom TV) to the watch by tapping it and turn it off, and try to re--pair your Airpods.
