In order to test whether coupons get applied to recurring subscriptions, you need to have an account that has a subscription placed and is waiting for the next renewal – you can create a dummy subscription order and it should work.
Use the referral link from that account in a new incognito window (or in a new browser) to make a new purchase so that the referrer can earn a coupon.
When purchasing through a referral link, make sure that the email address and the street address are not used by another customer on the website as they need to be unique, or else the plugin will mark the order as a potential fraud to prevent abuse.
If the order was made through the referral link successfully, you will see it in the box to the right, on the order screen.
Also, the user whose referral link was used should have a coupon available to them under the My Account page.
If everything is working as it should, go to Tools -> Scheduled Actions. Now you need to manually run renewal for the user that just got a coupon, to see that the coupon gets applied to their order.
In order to find out which subscription ID you need to run, just find the ID of the dummy order you made with that account and that’s it. After clicking run, you should have a new order under the order screen and if you check the details of that order, you should see a coupon applied and that coupon will disappear from the My Account section.
Find out how you can benefit from Using the WooCommerce Referral Plugin with WooCommerce Subscriptions.