18.04 server - unable to locate package redis-server [duplicate]
As a commenter noted, the universe package was not enabled. Enabling with add-apt-repository did the trick:
sudo add-apt-repository universe
I had the same issue on "Ubuntu 18.04 LTS on Windows 10" and accepted answer doesn't work for me but this answer worked. It might help someone else.
sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"