Hi Rajesh,
This is the reason you are seeing Error on the Approve Purchase Orders tile on the Launchpad. You have to maintain the workflow task ID in the back end system under Customizing for Materials Management under Purchasing > Purchase Order> Approve Purchase Orders App > Specify Workflow Task IDs for Approve Purchase Orders App. Once you configure this you will see the count of pending purchase orders on the tile.
Regards,
Saurabh