https://www.polygon.com/pokemon-go/2019/2/7/18213862/pokemon-lets-go-wont-connect-shiny-meltan

Pokémon Go’s new shiny Meltan event has made players return to Pokémon: Let’s Go! in the hopes of grabbing a Mystery Box, the exclusive item that unleashes Meltan into the wild once opened. So it’s perfect timing that, just as this event began, players are having widespread issues getting the smartphone game and Switch game to connect — a big problem, as transferring between the two is the only way to grab the Mystery Box.

Finding shiny Meltan, the mythical Pokémon with a little nut for a head, requires players to connect Pokémon Go to Pokémon: Let’s Go!; trading a Pokémon from the former game into the latter’s special Go Park area will send a Mystery Box back to Pokémon Go. Opening one gives players a small window to hunt down as many Meltan as they can find in-game.