"Environment subfigure undefined" on journal submission

or you can try:

\usepackage{subcaption}
\captionsetup{compatibility=false}

I would suggest a work-around, if all else fails, as it is difficult to test this without going through the submission process yourself. In that regard, avoid using the subfigure environment altogether, and default to using a tabular structure for arranging your subfigures. While this comes with manual numbering of subfigures, it's not that difficult to maintain references manually.

Here's a minimal example that should show you the implementation:

\documentclass[smallextended]{svjour3}
\usepackage{lipsum}% Just for this example
\usepackage{graphicx}
\begin{document}

See Figure~\ref{fig:myfig}(a) or~(b). \lipsum[1]

\begin{figure}
  \centering
  \begin{tabular}{@{}c@{}}
    \includegraphics[width=.7\linewidth,height=75pt]{example-image-a} \\[\abovecaptionskip]
    \small (a) An image
  \end{tabular}

  \vspace{\floatsep}

  \begin{tabular}{@{}c@{}}
    \includegraphics[width=.6\linewidth,height=100pt]{example-image-b} \\[\abovecaptionskip]
    \small (b) Another image
  \end{tabular}

  \caption{This is a figure caption}\label{fig:myfig}
\end{figure}

\lipsum[2]

\end{document}