When playing Pokémon UNITE on the Nintendo Switch, save data is automatically linked to the the User's Nintendo Account, however, it is possible to also link a Pokémon Trainer Club account to your save data if desired.
Please follow the below steps depending on your scenario.
-Existing Pokémon UNITE account-
Settings > Account Settings > Pokémon Trainer Club
Here you can unput your Pokémon Trainer Club credentials.
-Setting up a new Pokémon UNITE account-
Login Screen > Pokémon Trainer Club
Here you can unput your Pokémon Trainer Club credentials.