Learn how to activate the coupon code field
This is probably because you have not marked the coupon code in the registration field.
1. Click on the settings button to the right of the distance/class.
2. Click "edit" and go to the registration field.
3. Scroll down to the coupon code and mark the box.
4. Make sure to do the same procedure for every class/distance if you have more than one class/distance.