A header entry is identified by its fully qualified element name, which consists of the namespace URI and the local name. All immediate child elements of the SOAP Header element MUST be namespace-qualified.
You see that the <Transaction> element is prefixed by a t (and : too, but that is not part of the prefix). The t is the namespace prefix which says that the <Transaction> element belongs to the namespace with the prefix t.
The error message you get is, as far as I understand, because a header in your SOAP request lacks this namespace prefix.