How to test coupons with recurring subscription?
In order to test out if coupons get applied to recurring subscription, you need to have an account that has subscription placed and is waiting for next renewal, just make dummy subscription order and it should work. Use referral link from that account in a new incognito window or in the new browser, to make a new purchase so that referrer can earn a coupon. When doing a purchase through referral link, make sure that the email address and the street address are not used by some other customer on the site, they need to be unique or else plugin will mark the order as potential fraud to prevent the abuse.
If the order was made through referral link successfully, you will see it in the box to the right, on the order screen.
Also, the user whos referral link was used, should have a coupon available to him under My Account page.
If all is good, go to tools -> Scheduled Actions. Now you need to manually run renewal for the user that just got a coupon, to see that coupon gets applied to his 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 order screen and if you check the details of that order, you should see a coupon applied and that coupon disappeared from the My Account section.