Some of my subscriptions can't get renewed. The error that shows up is "Payment provider is not enabled on the shop." Why is this happening?

Let me explain here that all renewals are being processed by Shopify, meaning that our app sends a request to charge the customer to Shopify, which then processed this request and returns us with either a success response or an error message. 

The error you see here is what we get back from Shopify when we try to charge this customer. The error "Payment provider is not enabled on the shop," indicates that you previously had a payment provider (e.g. Shopify Payments, Authorize.net, etc.) enabled in the shop. When the customer paid for the initial order, they paid through this payment provider. Then you have removed this payment provider from your Shopify shop. When we trigger the charge, Shopify tried to process it, but as you don't have this payment provider enabled anymore, it just returns an error message. 

To resolve this, just reconnect the payment provider you had previously connected back to your shop.