I don’t have a Rasberry Pi to test with, to see exactly which packages you need to install for the “virtualenv” which is missing.
Personally I use an alternate client, as it has a lot less dependencies, so (to me ) works much more easily on some devices. You can see a list of alternative clients here - My own preference is a bash one ( https://github.com/lukas2511/letsencrypt.sh or https://github.com/srvrco/getssl )