titlesec to scrlayer-scrpage

Question

I'm trying to get my document to be free of warnings. One of them is

Usage of package `titlesec' together(scrreprt) with a KOMA-Script class is not recommended.
Non standard sectioning command detected(titlesec) Using default spacing and no format.

I already changed the layout so I'm not using titleformat anymore. The code still does'nt work without \usepackage{titlesec} though.. What am I missing?

\documentclass[11pt, a4paper]{scrreprt}

\usepackage{blindtext}
\usepackage{xcolor}
\usepackage{titlesec}
%\usepackage[headsepline, automark]{scrlayer-scrpage}

% \titleformat{\chapter}[display]     % {command}[shape]
%   {\usekomafont{chapter}\filcenter} % format
%   {                                 % label
%   {\fcolorbox{black}{shadecolor}{
%   {\huge\chaptertitlename\mbox{\hspace{1mm}}\thechapter}
%   }}}
%   {1pc}                             % sep (from chapternumber)
%   {\vspace{1pc}}                    % {before}[after] (before chaptertitle and after)

\KOMAoptions{chapterprefix}

\renewcommand{\chapterheadmidvskip}{%
    \vspace{12pt}
}
\setkomafont{chapter}{\Large\color{black}}
\renewcommand{\chapterlineswithprefixformat}[3]{%
    \parbox{\dimexpr\linewidth-2\fboxrule-2\fboxsep}{%
        #2 \vspace{.9pc}
        \hrule height 1pt \vspace{2pc}
        #3 \vspace{1.2pc}\hrule height 1pt%
    }
}
\renewcommand{\chapterheadmidvskip}{}
\renewcommand{\chapterformat}{%
    \mbox{\LARGE\MakeUppercase{Kapitel}\Huge~\thechapter\filright%
    }
}

\begin{document}
    \chapter{test}
    \blindtext
\end{document}

Answer 1

As @campa mentioned in a comment \filright is defined by package titlesec.

Here are some additional suggestions: Do not redefine \chapterheadmidvskip (this is done twice in your MWE). Instead set innerskip using \RedeclareSectionCommand. Add \raggedchapter inside the \parbox in the redefinition of \chapterlineswithprefixformat.

\documentclass[ngerman]{scrreprt}% 11pt and paper=a4 are default

\usepackage{blindtext}
\usepackage{xcolor}
\usepackage{babel}
\usepackage[headsepline, automark]{scrlayer-scrpage}

\KOMAoptions{chapterprefix}

\RedeclareSectionCommand[
  innerskip=0pt,
  font=\Large\color{black},
  prefixfont=\LARGE
]{chapter}

\renewcommand{\chapterlineswithprefixformat}[3]{%
  \ifstr{#1}{chapter}{% <- change the layout for chapters
    \parbox{\dimexpr\linewidth-2\fboxrule-2\fboxsep}{%
        \raggedchapter% <- added (default meaning is \raggedright)
        #2 \vspace{.9pc}
        \hrule height 1pt \vspace{2pc}
        #3 \vspace{1.2pc}\hrule height 1pt%
    }%
  }{%
    #2#3% original definition for other levels with style=chapter
  }
}

\renewcommand{\chapterformat}{%
    \MakeUppercase{\chaptername}\Huge~\thechapter
}

\begin{document}
\chapter{test}
\blindtext
\Blinddocument
\end{document}