Getting Error 401--Unauthorized

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Hi All,I am getting the below error while invoking the SOAP web-service for creating a requisition.I am  passing the credentials at the header level ,but still i am getting the below error.

 

the request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.46) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity MAY include relevant