[Tex/LaTex] Error using \limits

Question

My minimal TeX file

    \documentclass{article}
    \usepackage{amsmath}
    \begin{document}

     \begin{align}\label{eq:time_marginal_condition}
        x \left\| x \right\|\limits_{L^{2}(\mathbb{R})}^{2}
     \end{align}
     \end{document}

I get the warnings

./test.tex:8: Limit controls must follow a math operator.
<argument> ...tion} x \left \| x \right \|\limits 
                                                  _{L^{2}(\mathbb {R})}^{2} 
l.8          \end{align}

./test.tex:8: Undefined control sequence.
<argument> ... x \right \|\limits _{L^{2}(\mathbb 
                                                  {R})}^{2} 
l.8          \end{align}

./test.tex:8: Limit controls must follow a math operator.
<argument> ...tion} x \left \| x \right \|\limits 
                                                  _{L^{2}(\mathbb {R})}^{2} 
l.8          \end{align}

./test.tex:8: Undefined control sequence.
<argument> ... x \right \|\limits _{L^{2}(\mathbb 
                                                  {R})}^{2} 
l.8          \end{align}

How can you get rid of such warnings in the align environment?

My unsuccessful attempt

\begin{align}\label{eq:time_marginal_condition}
            x \left\| x \right\|
            \limits_{L^{2}( \texorpdfstring{\mathbb{R}}{} )}^{2}
\end{align}

Answer 1

Never use align for a single equation. But it's not the cause for your problems. One is that you don't load amssymb, so \mathbb isn't defined. The second is that \limits doesn't make sense.

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

\begin{document}
\begin{equation}\label{eq:time_marginal_condition}
  x \lVert x \rVert_{L^{2}(\mathbb{R})}^{2}
\end{equation}
\end{document}

Note that you shouldn't be using \left\| and \right\| unless you need the delimiter to grow.

The \limits keyword can be used after math operator such as \sum or \int, but makes no sense after \rVert or \right\|.