Removing block preview & read more at the end of the post

When I add the code into the “Edit code” section of the site it doesn’t work. Nor has any code for some reason. When I add it to the “Custom CSS” section from Online store>Themes>Customise>Theme settings>Custom CSS, the code does work but then doesn’t allow me to save the changes.

It says: “Online Store editor session can’t be published”

I am using Prestige 5-0 for context.

Any idea on how to fix this?