-2

How can I solve this problem?

enter image description here

I have so many similar to this problem.

2

You just need to add . after \right even if there is nothing to show in the typeset. This informs TeX engine that the brackets now ended. Check this code for example:

\begin{equation}
P_{e, \thinspace WT} =
\left \{
%
    \begin{aligned}
        %
        & 0  &   & v_{w} \thinspace < \thinspace v_{ci} 
        \\
        & P_{GCR} (v_{w}) \quad   & v_{ci} \thinspace \leq \thinspace &  v_{w} \thinspace \leq v_{r} 
        \\
        & P_{r, \thinspace WT}   & v_{r} \thinspace \leq \thinspace  &  v_{w} \thinspace \leq v_{co}
        \\
        & 0  &   & v_{w} > \thinspace v_{co}
        %
    \end{aligned}
%
\right
%
\label{eq:WTPSC}
\end{equation} 

Without the dot after \right, you will get a similar error to that shown above. Use the following code (the same as above but with \right.):

\begin{equation}
P_{e, \thinspace WT} =
\left \{
%
    \begin{aligned}
        %
        & 0  &   & v_{w} \thinspace < \thinspace v_{ci} 
        \\
        & P_{GCR} (v_{w}) \quad   & v_{ci} \thinspace \leq \thinspace &  v_{w} \thinspace \leq v_{r} 
        \\
        & P_{r, \thinspace WT}   & v_{r} \thinspace \leq \thinspace  &  v_{w} \thinspace \leq v_{co}
        \\
        & 0  &   & v_{w} > \thinspace v_{co}
        %
    \end{aligned}
%
\right.
%
\label{eq:WTPSC}
\end{equation} 

You will get:

enter image description here

Which means after finishing all of the cases close the set of equations, but not with another curly bracket - simply close it with the "nothing" bracket!

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