If you are working on code to merge two entity records and come across “Cannot specify child attribute retrieve” error; and you have referred SDK/MSDN documentation. You probably have kept UpdateContent property optional and havent set it. UpdateContent is a required property though the documentation says it is not.
This property is optional in case of merging incidents only.
You just have to set Entity object to UpdateContent property which has attributes to be updated to the merged record.
Entity contact = new Entity("contact"); contact["firstname"] = "Crm"; contact["lastname"] = "Xpress";
// merging contact records
MergeRequest merge = new MergeRequest
// SubordinateId is the GUID of the contact merging.
SubordinateId = contactId,
Target = target,
PerformParentingChecks = true,
UpdateContent = contact // Entity Object to set the content you want updated on the merged contact