Error “! Missing delimiter (. inserted).”

Question

The code below produces the error

$=-\frac { \pi  }{ 6 } \left[ { \left \frac { { u }^{ 3 }} { 3 }  \right| }_{ 0 }^{ 6 } \right]$    

Help please

Answer 1

Your code isn't easy to parse because you quite a few curly braces even though they aren't needed and end up creating significant clutter. Also, don't rely too much on auto-sizing brackets via \left and \right; instead, try to do what's visually, i.e., typographically, appropriate.

\documentclass{article}
\begin{document}
% left-hand side expression uses \left and \right
% right-hand side exression uses hand-sized "fences"
$-\frac{\pi}{6} \left[ \left. \frac {u^3}{3} \right|_{0}^{6} \right]
=-\frac{\pi}{6} \Bigl[ \frac{u^3}{3} \big|_0^6 \,\Bigr]$ 
\end{document}