You need to make sure that the table is not so wide that there is no room for the section head, but if there is room you can position a heading in the space, if you need that

\documentclass[a4paper, 12pt, twoside=true, openright, headsepline, pointlessnumbers]{scrbook}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english,ngerman]{babel}
\usepackage[backend=bibtex,style=alphabetic]{biblatex}
\addbibresource{citavi_lit.bib}
\usepackage{longtable}
\usepackage{lscape}
\usepackage{booktabs}
\begin{document}
\begin{landscape}
\begin{picture}(0,0)
\put(550,10){\rotatebox{-90}{\parbox{5cm}{\section{Test section}}}}
\end{picture}
\begin{longtable}{lll}
\caption{test}\\
\label{tab:test}\\\toprule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
\textbf{1} & \textbf{2} & \textbf{3} \\\midrule
1 & 2 3 \\\bottomrule
\end{longtable}
\ref{tab:test}
\end{landscape}
\end{document}
longtable
has no[H]
option (I really should make that give an error)\pictureref
for example, but also\toprule
,landscape
andlongtable
(although I could guess which packages you used for those)\rotatebox{-90}{\parbox{5cm}{\section{...}}}
but I don't think I'd do that.