Verify the Sales order header workflow. It should be waiting at sales order line close. now navigate to Sysadmin>>workflow>>administer workflow>>status monitor on the new webpage enter the item type as OEOH and item key as the header workflow ID and find.
Now click activity history and select the wait function and retry>>submit>>Okay
Now again go to the sales order header and verify the header status it should be complete.
In case the order is very old and the workflow has been purged then we will need a data fix to close the Order header.