The fix is as you describe it: go to python backstage and activated arcgispro-py3 in the `Manage Environments` dialog. You can also this at the command line : proswap arcgispro-py3 . At that point you should not get a blank `Python Package Manager` page.
What command line are you talking about?
Python window works:
But field Calculate Field is unavailable...
Edited moments later:
In a cmd window I cd into the C:\Program Files\ArcGIS\Pro\bin\Python\Scripts directory and run proswap arcgispro-py3 and I am returned this message:
The environment "arcgispro-py3" contains multiple NumPy installations based on its conda metadata. This typically is caused by upgrades or reinstallation, and should be resolved before trying to use the environment with ArcGIS Pro.
Another day, another problem to solve...
Edited again...
With conda list -n arcgispro-py3 I see that there are two versions of numpy installed (v 1.13.3 & 1.14.2). I also see two instals of several other packages:
notebook
pandas
etc
etc
Some have the the same version, some don't... How jacked can I make this?!
That should just about do it....