Hello Paul,
Thank you for reporting this to us!
I will try to replicate this issue with my conda environment to see how it can be corrected.
In the meanwhile, from the command line, you can install the matplotlib version it asks for with this command:
conda install matplotlib=3.2.0
or, if it still doesn’t work, you can go back one version with this:
conda install matplotlib=3.1.3
or
conda install matplotlib=3.1.0
This should help in bypassing the error and the installation should continue as normal.