When adding images to wiki.js file uploads must be enabled.
How to Enable File Uploads in MediaWiki
Use FileZilla to modify the file: LocalSettings.php
Using an FTP client or cPanel’s File Manager, locate and edit the file named LocalSettings.php. This file, by default, is located in the root directory for your MediaWiki installation.
https://www.inmotionhosting.com/support/edu/mediawiki/how-to-enable-file-uploads-in-mediawiki/