This article covers
Why do I receive the error “Customer who has received gift details is not supported to copy from one site to another”?
Currently, this error is thrown when one attempts to merge customers in which one of them or both customers have a gift subscription under him/her.
This is because the merging of customers with gift subscriptions is currently not possible as a gift subscription is treated as a different entity than a subscription.
This is the current intended behavior where one cannot merge two customer records with a gift subscription. If the gift subscription is deleted under the customer record, the merging can happen without any issues. However, we do not recommend the deletion of the gift subscription unless you are comfortable with it.
Related articles and Documentation
How can I merge two customer records in Chargebee?
How can I setup Gift subscriptions using Checkout V3?