center is not a floating environment, hence the error. You need the figure environment instead, i.e.
\begin{figure}
\centering
\includegraphics{..}
\caption{..}
\label{..}
\end{figure}
I'm assuming only one of the texts need to be part of the numbered caption, for the other just add some text in the appropriate location, e.g.
\begin{figure}
Lorem ipsum dolor sit amet, etc., etc.
\centering
\includegraphics{..}
\caption{..}
\label{..}
\end{figure}
If you don't need the image to float, you can continue using center instead of figure, but add \usepackage{caption} to the preamble, and use \captionof{figure}{caption text \label{...}} instead of \caption. Note that the \label has to be placed after or within the \caption, otherwise cross-references wont work, see e.g. Why does an environment's label have to appear after the caption?
\documentclass{article}
\usepackage{caption}
\begin{document}
\begin{center}
\includegraphics{..}
\captionof{figure}{...\label{..}}
\end{center}
\end{document}
As pagebreaks are allowed inside a center environment, you could here end up with image and caption on separate pages, which is probably not wanted. An alternative is to use a minipage instead of center, e.g.
\documentclass{article}
\usepackage{caption}
\begin{document}
\noindent\begin{minipage}{\textwidth}
\centering
\includegraphics{..}
\captionof{figure}{...\label{..}}
\end{minipage}
\end{document}
The \noindent is to remove the standard paragraph indentation.