I recently updated to El Capitan. When setting up one of our email accounts (a gmail account) in Mail, I received this alert: "You are already signed-in to the account". But of course the account does not appear on the accounts list in Mail. How can I correct this?
MacBook Pro (15-inch Early 2011), OSX El Capitan
I experienced this when setting up a new MacBook. The Internet Accounts were registered and logged in, but no services (Mail, Contacts, etc.) were enabled for syncing. Just go to System Preferences / Internet Accounts. Select (don't delete!) one account, and select the checkboxes for Mail and whatever else. That account should then appear in the Mail folders et al.
Go to System Preferences and open Internet Accounts there. If the account is in the list, delete it. Then re add it.
If it doesn't appear in the list, try adding the account in Other type account. You may still get a failure, but it might force a hidden account to show in the list. You have apparently signed into that account using some of it features such as in google maps, contacts, calendars, etc.
You're Welcome
Thank you very much.
Mail won't let me set up an email account.
