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I was getting this issue as well after following the same tutorial. I found I had an older version of redis previously installed that had created a redis-server.service file which specified an alias for itself of redis.service.
I ran sudo apt-get purge --auto-remove redis-server and then ran back through the tutorial steps. It’s possible I could have gotten away with just re-inserting my redis.conf file, since it was gone after the purge. But given that I redid everything, I couldn’t say for certain.
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