Font not installed
A font requested with fontspec (or a font package) is not available on this machine.
Fix
Use a font shipped with TeX Live, or add the font files to the project and load them by file name.
Example
Reproduced on TeX Live 2026 by CI · lualatex · corpus/fontspec-font-not-found
Font not installed (fontspec)
main.tex\documentclass{article}
\usepackage{fontspec}
\setmainfont{Helvetica Neue Pro}
\begin{document}
Hello.
\end{document}\documentclass{article}
\usepackage{fontspec}
\setmainfont{TeX Gyre Heros}
\begin{document}
Hello.
\end{document}main.log! Package fontspec Error:
(fontspec) The font "Helvetica Neue Pro" cannot be found;
(fontspec) this may be but usually is not a fontspec bug.
(fontspec) Either there is a typo in the font name/file, the
(fontspec) font is not installed (correctly), or there is a
(fontspec) bug in the underlying font loading engine
(fontspec) (XeTeX/luaotfload).
For immediate help type H <return>.
...
l.4 \begin
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