How to show a Drupal Commerce coupon system on the cart page?
Basic rule of thumb using any module in Drupal is first read Documentation or Readme or Install files inside Drupal module.. That will tell you how the module works... Go through Project page for full details


For fixed amount discount use Commerce coupon fixed amount


For Example - Readme file explains how to create %off coupon

Example - Create a '10% off' coupon

1) Install modules

Install:
   - Commerce Coupon
   - Commerce Coupon UI
   - Commerce coupon percentage

2) Create a 'Percentage coupon' coupon

Go to: Home > Administration > Store > Coupons

Click 'Create Coupon'

Choose 'Create Percentage coupon'
   (if you have no other coupon types clicking 'Create Coupon' will redirect to
    the 'Create Percentage coupon' form)

Create a percentage coupon with the following:


Coupon Code: 10%OFF
Number of Uses: 1 (change as necessary)
Percentage Amount: 10
Active: Yes

Save


3) Enable the 'Coupons' area on one of your Commerce checkout panes

Go to: Home > Administration > Store > Configuration > Checkout settings

Move the 'Coupons' item into one of the checkout panes

Save

4) Test it!

Please do some basic research before posting questions like above .. Otherwise they will be put onhold :-)