You can use the payment code on your rewards account to pay for all (or part) of the first order in a NEW subscription. Note that you will need to provide a credit card when creating a subscription for the future payments.
You cannot use your loyalty rewards balance to pay for the renewals of the subscription, renewals of subscriptions can only be paid with a regular credit card. This is a limitation of the platform that we are unable to change. Alternative forms of payment such as: gift cards, rewards points, HSA or FSA funds, cryptyo currency, etc… are not regular credit cards so the shopping cart system that we use cannot automatically charge them for renewing subscription orders.