To stop a user from selecting a delivery date from before the purchase orders creation date please follow the below steps to update the role permissions.
Click into the Admin Cogs
icon along the top bar.Click into Organisation Administration, and into Roles.
Edit the required role and click into the Permissions tab.
Click into the sub section Order Management and check / uncheck the permission Restrict Delivery Date Adjustments.
Scroll to the bottom and click Update to save any changes.
