Cannot edit a record in Order lines (SalesLine). An update conflict occurred due to another user process deleting the record or changing one or more fields in the record.

Some times when we perform delete or update actions then we might get error like 

"Cannot edit a record in Order lines (SalesLine). An update conflict occurred due to another user process deleting the record or changing one or more fields in the record."

Solution for this error is just add reread() before update statement.

salesLine.Reread()

salesLine.Update();

 Reference :Refer to this