# Undefined control sequence error \end{gather}

\documentclass{article}
\usepackage{mathtools,amssymb}

\begin{document}

\begin{gather}
\begin{split}

(1-\underset{(0.034)}{0.060}{\textit{B}}^{12}-\underset{(0.032)}{0.123}{\textit{B}}^{2(12)}){y}_t &=(1-\underset{(0.016)}{0.878}\textit{B}^{12})(1-\underset{(0.012)}{0.969}\textit{B}){e}_t\\

\end{split}

\notag\\

\end{gather}

\end{document}

• Welcome to TeX.SX! Please make your code compilable (if possible), or at least complete it with \documentclass{...}, the required \usepackage's, \begin{document}, and \end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. Jun 15 '17 at 0:24

Don't use blank lines in math mode, post a complete MWE, and please don't tell me that Box-Ljung is still being taught......

\documentclass{article}
\usepackage{mathtools,amssymb}

\begin{document}

\begin{gather}
\begin{split}
(1-\underset{(0.034)}{0.060}{\textit{B}}^{12}-\underset{(0.032)}{0.123}{\textit{B}}^{2(12)}){y}_t &=(1-\underset{(0.016)}{0.878}\textit{B}^{12})(1-\underset{(0.012)}{0.969}\textit{B}){e}_t\\
\end{split}
\notag\\