Vrftjkry

I've a simple formula:

documentclass{article}



usepackage{mathtools}



newcommandnorm[1]{leftlVert#1rightrVert}



begin{document}

    begin{equation*}

        displaystylesumlimits_{mathclap{j in {1,2}}}~frac{q^{0} - q^{j}}{{(norm{q^{0} - q^{j}}^{2} - d^{2}})^{2}}

    end{equation*} 

end{document}

After adding left and right commands to make the parenthesis of the denominator bigger, LaTeX generates the output as follows:

documentclass{article}



usepackage{mathtools}



newcommandnorm[1]{leftlVert#1rightrVert}



begin{document}

    begin{equation*}

        displaystylesumlimits_{mathclap{j in {1,2}}}~frac{q^{0} - q^{j}}{{left(norm{q^{0} - q^{j}}^{2} - d^{2}}right)^{2}}

    end{equation*} 

end{document}

but there is an error like Extra }, or forgotten right. ...orm{q^{0} - q^{j}}^{2} - d^{2}}right)^{2}}. The code is seamlessly compiled without those left and right commands.

You have too many useless bits in your code

• inside equation, displaystyle and limits are already in force


• additional braces in the denominator can be disruptive of spacing


• 
  mathclap is not needed (and is wrong, as the need for ~ shows)

Here are my proposals (I'd go with the first one). Note that with my definition of norm, norm* is essentially the same as yours.

documentclass{article}



usepackage{mathtools}



%newcommandnorm[1]{leftlVert#1rightrVert}

DeclarePairedDelimiter{norm}{lVert}{rVert}



begin{document}



subsection*{Good}



begin{equation*}

sum_{j in {1,2}} frac{q^{0} - q^{j}}{(norm{q^{0} - q^{j}}^{2} - d^{2})^{2}}

end{equation*}



subsection*{Possibly preferable}



begin{equation*}

sum_{j in {1,2}} frac{q^{0} - q^{j}}{bigl(norm{q^{0} - q^{j}}^{2} - d^{2}bigr)^{2}}

end{equation*}



subsection*{Possibly preferable again}



begin{equation*}

sum_{j in {1,2}} frac{q^{0} - q^{j}}{bigl(norm{q^{0} - q^{j}}^{2} - d^{2}bigr)^{!2}}

end{equation*}



subsection*{Disputable}



begin{equation*}

sum_{j in {1,2}} frac{q^{0} - q^{j}}{left(norm*{q^{0} - q^{j}}^{2} - d^{2}right)^{2}}

end{equation*}



end{document}

Grouping error in your code, below is the updated code:

begin{equation*}

    displaystylesumlimits_{mathclap{j in

    {1,2}}}~frac{q^{0} - q^{j}}{{left(norm{q^{0} - q^{j}}^{2} - d^{2}right)}^{2}}

end{equation*}