Troubleshooting
Command not found
The tool is not installed, or not on your path. Confirm with the version command from part one. If the version command works in one terminal and not another, it is your path, not the install — reopen the terminal first.
Wrong version
Confirm the installed version against the one in Before you start. A major version released after this series was written is the single most common cause of a step that "used to work".
Permission denied
You are writing somewhere you may not, or running as the wrong user. Resist the temptation to escalate privileges to make it go away — that turns a clear error into a file only an administrator can edit later.
It works locally but not deployed
Almost always a missing environment variable, a different runtime version, a path that only exists on your machine, or a file that was never committed. Check those four before anything else. See part three.
Nothing happens at all
Check you are running it from the directory you think you are, and that the file you edited is the file being read. Both are embarrassing and both are extremely common.
An error message you cannot parse
Read the last line first, then the first. The middle of a stack trace is usually somebody else's code. Search the exact text, in quotes, before reasoning about it.
Then re-run the check from part one. Two changes and one improvement means you now have two candidate explanations and no way to choose between them.
Still stuck#
Fill this in: where to ask, and what to include — the command, the exact error text, the version, and what you already tried.