Currently, we are using Custom App for development. We are facing an 403 Forbidden issue (ActiveResource::ForbiddenAccess (Failed. Response code = 403. Response message = Forbidden.)) when we are trying to hit ShopifyAPI::RecurringApplicationCharge.current api. So, anyone could you please let us know what we need to do here to rectify this.
There are a number of different possibilities as to why you are getting a 403 response.
If the shop you are testing on already has a recurring application charge for this app set up, you will not be able to create additional recurring application charges for this app on this store, as outlined in the documentation here: https://shopify.dev/docs/admin-api/rest/reference/billing/recurringapplicationcharge#updating-2020-0...
Additionally it is important to note that you cannot use the Billing API for private or custom applications. The Billing API is only usable in Public Applications as noted in the documentation here: https://help.shopify.com/en/api/tutorials/adding-billing-to-your-app
In order to investigate this issue further, we will need the x-request-id header of a more recent request that is giving you the same http response.