FK of the group <group> and PK of the parent group <group> should both either have an input field or not.
Internal error. If you connect the Primary Key of the root group, then the Foreign Key of all the immediate child groups must be connected, unless the child group fields do not have input values. If the Primary Key is not connected, none of the fields may be connected.
Validate the mapping and run the session again. Or contact Informatica Global Customer Support.