You say, "when I create a new project," what about opening an existing one? Same thing?
It may be something got crossed when installing one of the Python distributions. To start, I would check the registry to see what the values are for the ArcGIS Pro Python environment and make sure they are either set correctly or the files exist on the local hard drives:
C:\Users\bixb0012>reg query HKLM\Software\ESRI\ArcGISPro HKEY_LOCAL_MACHINE\Software\ESRI\ArcGISPro InstallDir REG_SZ C:\Program Files\ArcGIS\Pro\ .... PythonCondaEnv REG_SZ arcgispro-py3 PythonCondaRoot REG_SZ C:\Program Files\ArcGIS\Pro\bin\Python ....
There is a utility in the %ProInstall%/bin/Python/Scripts directory called 'proswap.bat' - you can use this batch file with an env name as an argument to automatically switch the necessary registry keys. Since there is no environment in the drop-down, I'm wondering if the %ProInstall%/bin/Python/envs/arcgispro-py3 folder still exists? The UI should be able to read the package information from that folder without the registry being set.
You may also want to check the PATH variable to ensure the Anaconda install isn't stealing your conda commands - currently we require envs to be within the Pro folder structure, so if they are in the Anaconda envs directory they won't work with Pro. They can exist side by side you just need to be mindful of the PATH.
Thanks everyone for the extra info. It was due to Anaconda setting the PATH variable.
I actually fixed it by installing the beta v2.0 of Pro and this corrected the PATH for me.