Hello
The error you are getting is the standard error and if there is any order (Production) exists in the system, you cannot delete the sales order line item.
Try the mentioned option
Option 1. Go to CO02, Mark your production order delete.
Than go to the reason of rejection tab, put the rejection reason at the line item level, and then tries to delete the line item on the same tab.
Option 2. Delete the schedule line against the sales order line item. This will make the sales order line item quantity as 0.
Hope this may help
Regards
SS