If you get an SSL error when trying to install PyXLL then the reason is usually because the root certificates in your Python environment are out of date.
Updating the "certifi" package to the latest version will usually solve this problem. You can do this using pip as follows:
pip install --upgrade certifi
Or if you are using conda you can use:
conda update certifi
If you are having problems updating the root certificates used by Python then a simple solutions to simply install PyXLL manually as that will avoid the problem altogether!
Installing PyXLL Manually
To install PyXLL manually download PyXLL from https://www.pyxll.com/download.html and make sure you select the right version of Python and Excel, and note that you can only use 64 bit Excel and 64 bit Python or 32 bit Excel and 32 bit Python, you can't mix them.
Then unzip the downloaded zip file to a folder and edit the pyxll.cfg file inside and set "executable" to the path to your python executable (type "where python" on the command line to find it). Make sure you remove the ";" from the start of the line as that's a comment character.
Now start Excel and go to Options -> Add-Ins -> Manage Excel Addins and browse to the pyxll.xll file that you have unpacked from the zip file. If Excel asks you if you want to copy this to your add-ins folder at this stage press "No".
And that's it! If everything has gone well PyXLL will now be installed.
You can find more detailed instructions on this procedure here https://www.pyxll.com/docs/userguide/installation/manual.html.