[Tex/LaTex] How to discard the following error (undefined control sequence)

Question

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}

Answer 1

Perhaps it is something like that you want?

\documentclass[11pt,a4paper]{article}
\usepackage{blindtext}
\usepackage{mathtools}
\DeclareMathOperator{\sgn}{sgn}
\begin{document}

\begin{flalign*}
+ba-ba& = +b × (+a-a)\\
& = (+a-a) × (+b),\\
& = -b × (+a-a),\\
& =(+a-a) × (-b),\\
\shortintertext{whence}
0 & =(+b) × 0=0 × (+b)=(-b) × 0=0 × (-b);\\
\shortintertext {or briefly}
& \begin{gathered} b × 0=0 × b=0. \end{gathered}
\end{flalign*}

\end{document}