In case you want to apply a general discount for the whole quotation/sale order, you can use promotion programs. 

1/ Activate the "Coupons & Promotions" feature in the sales app settings. 
2/ Create a new "Promotion program" where you indicate 
            a. Program name (e.g. 5% discount)
            b. Apply on all customers and all products that can be sold

            c. Select "Use a code"
            d. Define the code (e.g. 5) 

            e. Indicate the % to apply

See below screenshot 

3/ How to use it? 

                a. Create a new quotation and add products

                b. Click on "Coupon" and enter the coupon code (as defined earlier in the promotion program) 

And it should give you this : 

Warning!! This entry is only to be used in case you want to use a discount to be display on 1 line and which is a discount to be applied on the whole quotation. 
In case you want to show 5% discount on each line (instead of a new line), just use the usual "discount" field on the Quotation line.