Hi Jeff
Have a loog at the help on working with different authentication schemes | ArcGIS for Developers for the different login options. This worked in my case.
It could be that you are set up so you just need to enter the url to your subscription
gis = GIS("https://xxx.maps.arcgis.com")
and have Integrated Windows Authentication using NTLM or Kerberos (this works in my case) or you can for example use
gis = GIS("pro")
to use the login from your current ArcGIS Pro session.
From the website referenced above:
Using the pro
authentication scheme, scripts can get an instance of the GIS class representing the active portal in ArcGIS Pro without requiring the user to pass their credentials in a second time. In this mode, users can leverage the Pro app to login to the portal and their scripts can use whichever Portal is currently active. This mode can also serve as a bridge for users with advanced authentication scenarios like IWA using NTLM or Kerberos or Smart Card where signing in with credentials may not be possible or desirable.