There must be another field marked as required on the section. I would recommend checking all fields, saving and then resubmitting.
If this does not work, in order to resolve the issue that you are experiencing a thorough review of all of the factors contributing to the issue is required. Please submit a Service Request (SR) so that one of our Customer Support technicians can work with you on this. You can do so by contacting Pegasystems Global Technical Support through My Support Portal.