Can I add discount on total bill?

Yes, But you have to enable an option from configuration.

• Go to Setup > Configuration > Inventory Setting.
• Enable a option “Enable Bill Discount in Percentage”.If you want to give discount as amount you can also enable an option.”Enable Bill Discount as Fixed Amount”.
• Then you can see Item Discount(%) at the bottom of purchase transaction.You can enter value in percentage.
• If you want to gives discount as amount you have to enter the value in “Item discount(Rs)”.