So it seems to me that your are using some SSO solution at work. Probably it's based on AD so when you log on to your workstation and authenticate against AD this information is used later to authenticate against SAP. In this scenario SAP does not know AD password. It trusts AD and accepts whatever comes from AD. It's also possible that your SAP account does not have password maintained in SAP and it can also be disabled for username/password logon. Hence your only option to log on is to use SSO. Username/password will not work.
Cheers