AssociationGroup with Name '{0}' on Entity (External Content Type) with Name '{1}' and Version '{2}' in Namespace '{3}' is ...

AssociationGroup with Name '{0}' on Entity (External Content Type) with Name '{1}' and Version '{2}' in Namespace '{3}' is invalid as it is not defined on the destination Entity, or a reverse Association doesn't have the destination Entity as the only source.