In the screen capture below, the red rectangles highlight the possible causes of the "incomplete payment field" error.
1. The payment amount of a form-wide charge is missing.
2. The purchase field hasn't been configured, as indicated by the "Select Field" prompt.
3. The payment amount of a payment field is missing.
Note there is a dedicated forum for payments questions - http://forums.adobe.com/community/formscentral/payments