Make sure you have a Shopify subscription app installed on your store, like Ongoing.
Subscription information will be displayed in the following areas on your Shopify admin:
Subscription Information On the Product Page
On the product page in your Shopify admin, you can view a products subscription details.
Subscription details are shown in the Subscriptions section of the product page (if product or any variants have a subscription applied to it)
You can change your subscription product to be available only as a subscription. You can also offer both subscription plans and one-time purchase. Read our guide here.