A customer subscription is a charge that repeats. You might use it for a monthly service. It uses Stripe and is different from the M&M POS add-ons you buy.
Connect Stripe first. Open Stripe > Stripe Features and turn on Enable Recurring Subscriptions. Open Subscriptions > Add New Subscription Item. Enter Name, Price, Recurring Interval, and any Trial Days. Choose Save.
On Sell, choose the subscription and follow the signup steps. Explain the price, repeat payments, trial, and cancel rules to the customer before they agree.