If you are unable to enroll into a course with a Coupon, then check and make sure of the following:
The coupon you have, is applicable to the course you wish to enroll into.
Coupons are course specific so a coupon purchased for Std 11 Refresher will not work for the Confined Spaces Course.
The coupon is still valid.
Each coupon has an expiry date from the date of purchase. Check with your employer to make sure that the coupon is still valid.
Coupons may also be limited by the number of times it can be used. Check with your employer to be sure that it is still valid.