Why do I see the error "Non-selective query against large object type" in Salesforce when attempting to convert a subsequent recurring donation Lead record?

When attempting to convert a Lead record for a subsequent recurring donation, you may encounter this error:

Salesforce Error: sbxd.sbx_DonationsRecurringPaymentLead: execution of AfterInsert caused by: System.QueryException: Non-selective query against large object type (more than 200000 rows).

This can be resolved by submitting a ticket directly with Salesforce asking them toi index the field Subscription_Id__c on the Opportunity object in your Salesforce instance.