You are missing the Subject and Email blurb at the envelope level like below:
</ns:Tabs>
<ns:Subject>This is the subject</ns:Subject>
<ns:EmailBlurb>This is the test body of the message</ns:EmailBlurb>
<ns:AllowReassign>true</ns:AllowReassign>
but unless the multi-language feature is turned on for your account (demo or Prod) you will then get the following error:
<faultstring>A requested plan item is not enabled for this account. Plan item: SignInEachLocation</faultstring>
<faultactor>https://demo.docusign.net/api/3.0/dsapi.asmx</faultactor>
So you can contact support to ask them to enable the multi-language feature.