This even stumped a rep. from Nintendo. I can’t connect Pokemon Go and Let’s Go Pikachu!.
The cart is in the switch. Open Pokemon Go Settings is clicked on. I say yes to pair with Pokemon Go account. And the Switch hunts and hunts to connect but fails. This happens when the connection is ready and able on the phone game.
Are you trying to get the Meltan boxes? I suggest instead of using the Let’s Go games, set up and connect to Pokemon Home.