The likely cause of this, is the Event being a sign up only event and having a Payment Gateway Selected, and no Payment Options selected. 

To fix this, navigate to:
  1. onMessage > Approval > Event Registration
  2. Click Edit next to the Event Registration name
  3. On the General Info Tab, scroll down to the Payment Options Account to Use, change this to Do not Use credit Card Processing
  4. Click Save in the upper right