Which payment gateways does your app support?The auto-charging subscription rules require you to have the Shopify Payments, ShopPay, Apple Pay, Google Pay, Authorize.net, Stripe or at least PayPal Express set up in your shop. This way, you can accept subscriptions with an auto-charging functionality. You can read more about requirements here: https://help.shopify.com/en/manual/products/subscriptions/setup#eligibility-requirements. You can't use the auto-charging subscription rules without Shopify Payments, ShopPay, Apple Pay, Google Pay, Authorize.net, Stripe or PayPal Express. The app will alert you if you aren't eligible for the auto-charging functionality when you try to create/edit an auto-charging subscription rule.
To become eligible for using PayPal Express for subscriptions, you have to configure PayPal Express in your shop AND create at least one auto-charging subscription rule. When Shopify detects that, they will automatically submit your shop for approval by PayPal and send you an email with more info about that. After you get approved by PayPal, you will be able to start accepting subscriptions in your shop with PayPal Express.
If you can't use Shopify Payments, ShopPay, Apple Pay, Google Pay, Authorize.net, Stripe or PayPal Express, then you can still use the app in recurring invoices mode. This way, you just create a recurring invoices subscription rule and when your customers subscribe and the time comes to pay for their next recurring order, the app will send them an automatic email reminder with the checkout link where they can pay for the recurring order. The app WON'T automatically charge your customers for subscriptions created in this mode. The customers will go to a normal checkout in your store (Shopify Checkout) and will be able to use any payment method you have configured in your store.