If you are using any other plugin (or a theme) that uses a shortcode with the name 'popup', you will not be able to use this plugin."
If you know this already you should use more specific class name like "A-popup" or "Alligator-popup" to avoid conflict.
I have to edit the name to avoid conflict with my theme.
You would want to have a plugin that work with any themes.
Thanks for the plugin.