I Was having issues with this - Couldn't clone the defualy ArcPro Python environment so I used Conda outside out ArcPro to fix the issue.
I opened the default conda environment > opening this batch file:
C:\Program Files\ArcGIS\Pro\bin\Python\Scripts\proenv.bat
Then cloned the environment:
conda create --name arcpro_clone --clone arcgispro-py3
activate arcpro_clone
The environment was installed in:
C:\Users\<user>\AppData\Local\ESRI\conda\envs\arcpro_clone
following this it was possible to edit the environment with Conda directly
Set up a batch(.bat) to open this env more effeciently in future with:
@echo off
set PATH=%PATH%;C:\Users\<user>\AppData\Local\ESRI\conda\envs\arcpro_clone\Scripts
%windir%\system32\cmd.exe "/K" C:\Users\<user>\AppData\Local\ESRI\conda\envs\arcpro_clone\Scripts\activate.bat arcpro_clone
Cheers