On my own MediaWiki 1.35 installation the CodeEditor extension does not work (downloaded the newest CodeEditor and WikiEditor packages).
Here, on MediaWiki.org all extensions are installed (Special:Version), same situation, CodeEditor not appearing if you try to edit CSS, JS or Module pages, like: https://www.mediawiki.org/w/index.php?title=MediaWiki:Vector.css&action=edit
If you check Wikipedia, CodeEditor is working there: https://en.wikipedia.org/w/index.php?title=MediaWiki:Common.css&action=edit One difference could be, VisualEditor is not enabled on Wikipedia in contrast to MediaWiki.org?
What is status of this extension, if it is not even working here, on the official site?