Summary

The error message can come up when the merchant is on the Rise Plan or Performance plan and they are using an API to add a payment method to an existing customer record. It's a feature restriction as anything below Scale cannot have multiple cards added.

Solution 

As an alternative,  if the merchant wants to replace the Card in the customer details, using the same Endpoint, they’ll have to pass the "replace_primary_payment_source" as "true"(https://apidocs.chargebee.com/docs/api/payment_sources?prod_cat_ver=2#create_a_bank_account_payment_source)