Are all coupons not working or just some? If you are not sure, create a very simple coupon code, 1% discount, and try it. If it works, then it must be something in the coupon you create, in which case you can go to awocupon->configuration->error message tab and change the errors to something identifiable then try it again. If it is still not working, it could be the override was not installed for that, you can use this documentaiton:
The prestashop tab, and number 10
If you want me to look at it directly I can, for that please send a private message with temporary admin and ftp access.