Why does biblatex warn that

Keyword 'additional' not found on input line 33.


and, accordingly, not print the second bibliography?

\RequirePackage{filecontents}

\begin{filecontents}{references.bib}
@Article{EulerE,
author = {Euler, Leonhard},
journal = {Math.\ Psychol.},
year = {1776},
volume = {4},
pages={1--2718},
keywords={cited},
}
@Book{BourbakiPure,
author={Bourbaki, Nicolas},
title={A Course of Very Pure Mathematics},
year={1956},
publisher={Erehwon Press},
}
\end{filecontents}

\documentclass{article}

\usepackage[style=numeric]{biblatex}

\begin{document}

\textcite{EulerE} studied the constant $e$.

\printbibliography[keyword=cited,title=Cited Works]

\end{document}


biblatex (and indeed also classical BibTeX) only considers entries that were cited. If you want to add an entry to the bibliography without citing it, you can use \nocite{<key>}. With \nocite{*} you add all entries from your .bib files to the bibliography.

Note that it is not easily possible to \nocite only entries with a particular keyword: Is it possible to add entries to the bibliography based on keyword using Biblatex/Biber and within the document code?. Workarounds might be possible in certain situations, but they can not satisfy all desiderata.

\documentclass{article}

\usepackage[style=numeric, defernumbers=true]{biblatex}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Article{EulerE,
author = {Euler, Leonhard},
journal = {Math.\ Psychol.},
year = {1776},
volume = {4},
pages={1--2718},
keywords={cited},
}
@Book{BourbakiPure,
author={Bourbaki, Nicolas},
title={A Course of Very Pure Mathematics},
year={1956},
publisher={Erehwon Press},
}
\end{filecontents}

\begin{document}
\textcite{EulerE} studied the constant $e$.

\printbibliography[keyword=cited,title=Cited Works]
\nocite{BourbakiPure}

Since you have a numeric split bibliography, you should look into using defernumbers=true to make sure the numbering is increasing.