For subscriptions, Shopify only accepts the following 4 payment gateways:

1) Shopify payments

2) PayPal Checkout/Express

3) Authorize.Net

4.) Stripe

If you use any of the above payment gateways, you are eligible to sell subscription products on Shopify.

If you use something different to process credit card payments, you are ineligible for Shopify subscriptions.

But don't worry, you can easily fix this. Here is our article on How to Fix your Subscription Eligibility