A mandatory new due date is required when approving a requisition in R21a. The purpose is for an approver to set a due date for a requisition to be approved.
If this feature is not needed, admin can customize the approval screen (see this blog for customization for a group) and remove the checkbox (1), and new and current due dates (2 and 3). The screen below shows the REQ approval screen without the due date fields. An approver can just approve a REQ without the need to set a due date.