What to Do:
Navigate to Sign in.
Sign in with your Google Account. If you have multiple Google Accounts, be sure to select the account you use with YouTube.
After signing in, you may be presented with a list of Google identities associated with your account. Be sure to choose the account that is linked to your YouTube channel. If you select a Brand Account that doesn't have a YouTube channel, you won't be able to sign in.
Note: When launching the app, you’ll be prompted to select a Nintendo account to use. Accounts that are logged into the YouTube app can be accessed via any Nintendo account on the device.