when I try to Invite Attendees, I get a popup saying “Before you send a meeting invitation, you must first add an e-mail account to Outlook

In Calendar, when I try to Invite Attendees, I get a popup saying "Before you send a meeting invitation, you must first add an e-mail account to Outlook.  To add an account, on the Tools menu, click Accounts."   I already have 2 accounts in there but Calendar sees neither.  What's up and how do I fix this?