Hi Charlie,
What kind of authentication are you using for Enterprise? Also can you also try out a couple lines of python code for me. You can send me the results via a direct message.
Open a python window from the Analysis window in Pro.
Type in and run this code.
from arcgis.gis import GIS
GIS("Pro", verify_cert=False)
Send me the result. Just need to know if it created this successfully..
Next run this line
arcpy.GetSigninToken()['token']
it should print out a token. let me know what it does.
I checked with our developers.. and basically if it cannot get a token it will show that message.
Randall