# [Tex/LaTex] “! paragraph ended before \align was complete”

alignbrackets

The following is the piece of code that was working right before I made a few small minor changes.

\begin{align}
\langle \prod_{i=1}^{n} \delta_i ^{n_i} y^{m_i}_i  \rangle
= \sum _ {a_{i=1...n} = 0}^{n_{i=1...n}}
\sum_{b_{i=1...n} = 0} ^{m_{i=1...n}}
\prod_{i=1}^{n} \left [ ^{n_i}C_{a_i} ( \sum _ {\rho_{\{a_{1\leq i \leq n}\}}}
\prod _{B \in \rho_{\{ a_{1 \leq i \leq n} \}}
\langle \delta_i ^{\vert B \vert} \rangle _c ) ^{m_i}C_{b_i}
( \sum _ {\rho_{\{b_{1\leq i\leq n}\}}}
\prod _{B \in \rho_{\{b_{1\leq i \leq n} \}}}
\langle y_i ^{\vert B \vert} \rangle_c ) \right ] \langle
\prod_{i=1}^{n} \delta_i ^{n_i - a_i} y_i ^{m_i - b_i} \rangle _{nzl}
\end{align}

• Also can I get the innermost pair of $( )$ brackets to be large using \left and \right?

You're missing a closing curly brace to delimit the subscript of the third \prod expression.

In the code below, I've also (i) inserted a line break, (ii) increased the sizes of some of the fences, and (iii) replaced ... with ,\dots,. Note that the size of the fences is deliberately set so as not to span the subscripts and superscripts of the sum and product symbols. \documentclass{article}
\usepackage{amsmath}
\usepackage[margin=1in]{geometry} % use whatever page margins are right for you
\begin{document}
\begin{align}
\biggl\langle \prod_{i=1}^{n} \delta_i ^{n_i} y^{m_i}_i \biggr\rangle
&=
\sum _ {a_{i=1,\dots, n} = 0}^{n_{i=1,\dots, n}}
\sum_{b_{i=1,\dots, n} = 0} ^{m_{i=1,\dots, n}}
\prod_{i=1}^{n} \Bigg[
{}^{n_i}C_{a_i} \biggl(
\sum _ {\rho_{\{a_{1\leq i \leq n}\}}}
\prod _{B \in \rho_{\{ a_{1 \leq i \leq n} \}}}
\bigl\langle \delta_i ^{\vert B \vert} \bigr\rangle _c
\biggr) ^{m_i}\notag\\