MTurk Requester encountering error: 'There was a problem submitting your results for this HIT'

Hi,

I'm creating a survey in which workers respond to questions at a survey link and then provide a unique 5 digit survey completion code on MTurk.

When previewing the survey in MTurk as if I was a worker, and entering a survey code, I always get the following error: There was a problem submitting your results for this HIT. This HIT is still assigned to you. To try this HIT again, refresh the page. If this problem persists, you can contact the Requester for this HIT by clicking "HIT Details" above and then clicking "Contact This Requester" at the bottom of the pop up. To return this HIT and continue working on other HITs, click the "Return" button on the top or bottom of the right side of the page.

Does anyone know how to fix this? Do I need to change the source code to fix this? If so, how?

Thank you, Catherine S