Coupons can't be used for Subscriptions, coupons can only be used with Products.
For a Product, please check the Coupon settings, it should be as below:
and note that you must define which Products it can be used for (it can't be set to "no restrictions", or the setting following it won't appear).
Then the setting "Can be used for renewals (etc)" will display, which you need to enable as above.