Is there any way of adding a recurring coupon to a payment plan already in progress?

To add a recurring coupon to an existing payment plan, do the following:

1. Create a recurring coupon.
2. Go to WooCommerce > Payment Plans and open the payment plan.
3. Click the Apply coupon button.
4. Enter the coupon name and update the payment plan.

Note: you may also need to click the Recalculate button before updating the payment plan.