Please note that this error does not affect for I could always ignore it and proceed, BUT I cannot figure out what causes it to occur. Error occurs at the last line; \end{flalign}
\documentclass[11pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\DeclareMathOperator{\sgn}{sgn}
\begin{document}
\begin{flalign*}
+ba-ba&=
+b\times (+a-a)\\
&=(+a-a)\times (+b),\\
&=-b\times (+a-a),\\
&=(+a-a)\times (-b),\\
\end{flalign*}
\begin{flalign*}
&\unalign{\text{whence}}&& 0=(+b)\times 0=0\times (+b)=(-b)\times 0=0\times (-b);\\
&\unalign{\text {or briefly}}&& b\times 0=0\times b=0.
\end{flalign*}
\end{document}
Best Answer
Perhaps it is something like that you want?