This document throws an error Package scrbase Error: unknown option 'runin=false'.
scrbase.sty indicates this is KOMA-Script version 2026/02/02 v3.49.2 KOMA-Script.
\documentclass{scrreport}
\RedeclareSectionCommand[
beforeskip=1sp,%
afterskip=-1sp,%
runin=false%
]{chapter}
\begin{document}
\chapter{A Chapter}
Chapter text.
\section{A section}
Section text.
\end{document}
My understanding is, this option should be allowed as used in this M(N)WE.
Using up-to-date TeX-live on MacOS (Mx) if it matters...
runinoption is only available forsection, notchapter. What is the expected output?runinapplies only tosectionand "below"... and surprisingly it works :)