Logging in to Portal for ArcGIS with built-in ArcGIS logins via ArcGIS API for Python fails when disabling 'Allow users to sign in with their ArcGIS built-in account' in the portal security settings.
In ArcGIS Enterprise 11.x, administrators have the ability to disable the built-in Portal for ArcGIS accounts. If the SAML and built-in accounts are enabled, ArcGIS API for Python authenticates successfully. The other alternative is to use 'use_gen_token=True' with the built-in account information. ArcGIS API for Python does not support SAML logins.