There may be two reasons on your site that the customer is not getting the referral coupon.

1. The referee will get the coupon when referred user get signup using the referral and place the order then order status must be marked as completed only then the referee will get the referral coupon.

2. If you are using WooCommerce Points and Rewards plugin and We have also provided compatibility with this plugin. We have provided setting under WooCommerce > Settings > Referal > Points and Rewards
Here we have provided a setting "Enable Points instead of coupons". After enabling these settings then customers will get the points instead of the coupon.

You can check these settings once.