Forum Discussion
Can't Activate SharePoint Server Publishing Infrastructure for Site Collection
You are unable to do this for various reasons, the first being that scripting is disabled in the Modern UI. It is not recommended to enable publishing in Modern for multiple reasons, one being that Modern UI doesn't support MasterPages/page layouts.
I would think about avoiding mixing classic ui and modern ui, as it is going to give your end users a very inconsistent experience navigating between modern and classic pages. I would think about seeing if you can create similar functionality in Modern without using classic. If you cannot, I would create a classic SharePoint site instead for the wiki purposes.
Thanks for the reply.
We are simply trying to change the default text size for a web part. There doesn't seem to be any easy way to do this as a user must configure the text size each time for each new web part.
I appreciate the confirmation that publishing is not enabled in the Modern UI.