Charge 6 cannot be set up with existing Fitbit or existing Google account...

[BobLindemann]

Trying to assist my 91 year old Dad who is experiencing this exact issue. He has no clue if he had another Google account years ago - as far as he knows he did not. How can we tell what that account might is been?

[jboogie]

Sometimes these accounts are created with Google by accident because during the login that it prompts you with familiar accounts on your phone.

On the Fitbit app, login with his Google email and password, then go to settings or options and find delete account.

He'll have to wait for the Google account to be deleted from Fitbit it takes 7 days.

After it's been deleted, he can choose to migrate his account to his Google account.

[Rich_Laue]

@BobLindemann I'd suggest you help dad create a Google account.

Has dad been using a fitbit? If yes, convert dad's fitbit account to a Google Login with the newly created Google account. 

If dad does not currently have a fitbit account, you want to help dad create one, using the Log in with Google. 

I've also moved your post into its own thread for clarity