Dory is right. Emails cannot be approved without a subject line. Also, Email A/B Tests cannot be approved if the email does not have a subject. This is because when you approve an A/B Test it will attempt to approve the email if it is a draft.
I tried a little experiment with this... I tried creating an email without a subject line and then an A/B test. When I tried to approve the test, it errored out. I was not surprised due to the above.
I was a little surprised when I entered a quick subject line and tried again. The email and test were approved (expected), but the Champion I entered in the test was overwritten by the subject line I quickly entered in the email draft. Another reason you should approve your email (or at the very least give it a subject line) before you create your A/B Test.