When sending a declaration to the Customs Declaration Service (CDS) you may receive a Code 401: Unauthorized request message. In this case you will need to check that the authority, granted by HMRC, to allow your software to interact with CDS is valid, and your Government Gateway account details are ok.
See the article: How do I grant authority for the AEB software to interact with CDS, NCTS, GVMS, S&S GB and EMCS? for further details about granting authority.