Anhand des Links hab ich den Code verändert. Mein jetziges Problem, liegt vermutlich darin begründet, dass ich nicht wirklich weiß, wass ich da tue.
\documentclass[man,a4paper]{apa6}
\usepackage{xltxtra}
\defaultfontfeatures{Mapping=tex-text}
% Sprache
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new]{german}
\usepackage{blindtext}
\def\flushboth{%
\let\\\@normalcr
\@rightskip\z@skip \rightskip\@rightskip
\leftskip\z@skip
\parindent 1.5em\relax}
\title{Title}
\shorttitle{STitle}
\begin{document}
\flushboth
\blindtext
\end{document}
Das erzeugte PDF sieht gut aus. Blocksatz.
Aber Fehler werden dennoch geworfen.
! Undefined control sequence.
\flushboth ->\let \\\@normalcr \@rightskip\z
@skip \rightskip \@rightskip \l...
l.22 \flushboth
! Undefined control sequence.
\flushboth ...\rightskip \@rightskip \leftskip \z
@skip \parindent 1.5em\relax
l.22 \flushboth
! Missing number, treated as zero.
<to be read again>
@
l.22 \flushboth
! Illegal unit of measure (pt inserted).
<to be read again>
@
l.22 \flushboth
AED endfloat: Processing end Figures and Tables [1] (./block.aux) )
(see the transcript file for additional information)
Output written on block.pdf (1 page).
Transcript written on block.log.