I think what CarolineS​ said is probably the issue
created via the admin console with a username & password
Even if your community has SSO you can create a non SSO account in admin. Create new users ( - Lithium Community That is what you should use.
Also, if you are trying to do this on stage you will need to include the HT access username and password or ask support to whitelist your IP for stage, so you don't need it.
Also, not sure if you are on Aurora or Classic. I am only familiar with classic however, based on Matt's option 1 here Can someone walk me through authenticating and using Postman with Aurora? | Atlas I think it is the same for Aurora.