%Dokumentklasse
\documentclass[twoside,a4paper,11pt]{scrreprt}
% ============= Header & Footer =============
\usepackage[plainheadsepline]{scrlayer-scrpage}
\pagestyle{scrheadings}
\clearpairofpagestyles
% ============= Beginning of the Document =============
\begin{document}
\setheadsepline{0.0pt}
Text
\end{document}
StefanPackage scrlayer-scrpage Warning: Command deprecated!
(scrlayer-scrpage) Usage of `\setheadtopline', `\setheadsepline',
(scrlayer-scrpage) `\setfootsepline', and `\setfootbotline'
(scrlayer-scrpage) became deprecated with scrlayer-scrpage.
(scrlayer-scrpage) You should use KOMA-Script options
(scrlayer-scrpage) `headsepline', `headtopline', `footsepline',
(scrlayer-scrpage) or `footbotline' with values
(scrlayer-scrpage) `<line thickness>:<line length>' to setup
(scrlayer-scrpage) the line length and thickness, and
(scrlayer-scrpage) `\setkomafont' or `\addtokomafont' to
(scrlayer-scrpage) setup the colour on input line 14.
\usepackage[plainheadsepline, headsepline=0.0pt]{scrlayer-scrpage}