Go back to the Personal Info section and make sure you did not indicate that you were Married- if you are married on December 31st you cannot file as Single.
Are you using an old account that was used for past joint returns? If so, you should start a new account with a new user ID. It is too hard and messy to remove the embedded "spouse" stuff from an account.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**