System has not been booted with systemd as init system (PID 1). Can't operate

Instead, use: sudo service redis-server start

I had the same problem, stopping/starting other services from within Ubuntu on WSL. This worked, where systemctl did not.

And one could reasonably wonder, "how would you know that the service name was 'redis-server'?" You can see them using service --status-all

Instead of using

sudo systemctl start redis

use:

sudo /etc/init.d/redis start

as of right now we do not have systemd in WSL

If you are using Docker, you may try an image that has Ubuntu with System D already active with this command:

docker run -d --name redis --privileged -v /sys/fs/cgroup:/sys/fs/cgroup:ro jrei/systemd-ubuntu:18.04

Then you just need to run:

docker exec -it redis /bin/bash

and there you can just install Redis, start it, restart it or whatever you need.