The "Summary not found" error occurs mostly due how the content is being copied over from AO3. The entire webpage needs to be copied or the macro will not function correctly. To ensure you're copying correctly, please make be sure of the following:
You are copying only from the AO3 website - the formatting macro will not work with any other sites.
You have 'Entire Work' selected
You are copying the entire web page. You can use the shortcut ctrl + a (or cmd+a for Mac systems), to copy the full page, and then ctrl+c (or cmd+c) to copy.