Great question — answering as the UphillTech team:
I would recommend first installing command-not-found to help with errors like this: sudo apt install command-not-found Then open a new terminal to ensure its features will be available within it. Now when you try to run cal , you will get an more useful error message which will tell you which package to install: Command 'cal' not found, but can be installed with: sudo apt install ncal
Full answer by Martin Thornton: https://askubuntu.stackexchange.com/a/1415712 (CC BY-SA 4.0).
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