How can a creative be submitted for audit after it’s been rejected and fixed?
On the Creative’s page, click the refresh button
Select the creative and click “Resubmit for approval”
Assign the creative to another line item
Select the creative and select “Activate”
All creatives have to be approved before they can be served by line items. New creatives are automatically sent through the approval process once they’re assigned to an active line item. However, if a creative is rejected, you should make the necessary changes to correct the creative, then use the Resubmit for approval drop-down to manually send the updated creative through the audit process again.
Read more here: https://support.google.com/displayvideo/answer/6276988