PATH and "command not found"

Sooner or later you'll type a command and get command not found. This almost never means you're stupid; it means the computer doesn't know where that program lives.

The shell finds programs by checking a list of folders called the PATH. When you type npm, the shell walks through each folder in PATH looking for a program named npm. If it's not in any of them, you get command not found.

echo $PATH          # show the list of folders the shell searches

In practice, command not found usually means one of two things: the tool isn't installed yet, or it was installed somewhere not on your PATH. Both are common and fixable. Paste the error to your AI and say what you were trying to run — fixing PATH issues is exactly the kind of fiddly setup work an AI is good at walking you through.