Cartopy installation fails

I use Anaconda3 in a windows 10 environment. I try to install Cartopy package using the standard procedure :

conda -c install conda-forge cartopy

To succeed, the installation requires an old version of pystan (pystan 2.17) and tries to downgrade the already installed version (pystan 2.18). 

Cartopy installation fails with the following error :

ERROR An error occurred while uninstalling package ‘defaults::pystan-’.
PermissionError(13, ‘Access is denied’)

Can somebody suggest a solution ? Thanks,