You cannot.
Today Duo provides secondary authentication only. In order to use Duo you must have some existing source for primary credential authentication, such as Active Directory, OpenLDAP directory, Entra ID, Google Directory, Okta, OneLogin, Ping, etc.
After primary username and password is validated in your user identity store, Duo adds MFA and device and other security checks before users can access your Duo-protected applications.
Duo, not DUO.