Great question — answering as the UphillTech team:
Sometimes this happens because APT just doesn't know anything about what you're talking about, but that can be solved by running: sudo apt-get update After running this, typically the error goes away because apt and apt-get know how to do what I ask now.
Full answer by Wayne Workman: https://askubuntu.stackexchange.com/a/878509 (CC BY-SA 4.0).
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