\begin{equation}
\centering \Large \textbf{$\lambda_{c} = \frac{18.64}{B \times E^{2} }$ }
\label{eq1:Critical wavelength}
\end{equation}
\begin{center}
\captionof
{equation}{where B is the bending magnet magnetic field ( B = 1T ) , E is the energy of the booster}
\end{center}
Undefined control sequence in equation caption
captionserrorsundefined
Best Answer
Making a test document from your fragment
Produces the error
as blank lines are not allowed in math mode.
Deleting the blank lines you get
so deletimg
\Large
you get
as
\captionof
is defined by thecaption
orcapt-of
packages, but addingdefines
\captionof
command but as equations captioning is not set up you getas the equation already has an equation number it is hard to guess what formatting you would want for a separately numbered caption, so I suggest deleting this. Also
\centering
and\textbf
are doing nothing in this context, so I think you want: