Hi Chloe,
you are using Anaconda3, which by default includes python 3. Sen2cor requires python2.7, so you could install Anaconda2 or change the python version of your Anaconda3 installation (see https://docs.continuum.io/anaconda/faq#does-anaconda-include-python-2-or-python-3) . I recommend you to install Anaconda2, I think it should be easier.