Replacing Formats menu with editor-style.css

Hello,
In the settings of this plugin I have the option to “Create CSS classes menu”

It says I need to upload a css file named editor-style.css to my theme, and it will replace the Formats menu.
I have uploaded a file called editor-style.css to my child theme folder, but it is still saying:

“Disabled: A style sheet file named editor-style.css was not added by your theme.”

Is this a bug, or do I need to do something else to get it to work?

thanks for any help!