B.9 Environment ... undefined

LaTeX doesn't recognise the environment you have specified.

Check you have spelt the environment name correctly.
You will get this error if you do, say,
✘ \begin{docment}
instead of
✔ \begin{document}
If it's your own environment, check you have defined the environment before using it.
If the environment is defined in a package, check you have included the package using the \usepackage command.

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