If you get the error message Violation: “extensionTaxonomyDimensionNotAssignedDefaultMemberInDedicatedPlaceholder” it is because an element in the extension taxonomy hasn’t been assigned a default member.
In most cases this is an issue that needs to be handled by the software provider who helped creating the ESEF file, unless you use a solution where you create the entire taxonomy yourself. In that case, you need to assign the default member property to one of the dimension members in the extension dimension.