Looks like it my have been my child theme which is a custom theme from web-savvy-marketing:
“It’s actually the child theme.
Both the default sidebars are unregistered in the functions.php file.
unregister_sidebar(‘sidebar’);
unregister_sidebar(‘sidebar-alt’);
Simple sidebars only works with those sidebars so if they are unregistered, nothing will appear in the sidebar selection on posts or pages.”
I deleted the two “unregister” lines above from my functions.php file and the Simple Sidebars plugin is working again with drop downs!