I added add_theme_support( ‘wp-block-styles’ ); and the folder templates with, at least, the file index.html to allow it. With a virgine theme, I’m able to do it but with your theme, no. It’s a custom version of your theme but normally, It is suppose to work well.
What version of Sage?
There is an issue/PR for Sage 10 + FSE support:
There are still some inconsistencies with the Gutenberg plugin.
What kind of issue occurs? Does the Full Site Editing part in backend doesn’t appear?
Are FSE block templates or block parts not loaded?
Mostly, I dont have the Editor in themes that allow me to handle the template, etc.