Whoops! The default skin for your wiki, defined in $wgDefaultSkin as cavendish, is not available.
You have no installed skins.
If you have just installed or upgraded MediaWiki
You probably installed from git, or directly from the source code using some other method. This is expected. MediaWiki 1.24 and newer doesn't include any skins in the main repository. Try installing some skins from mediawiki.org's skin directory, by:
Downloading the tarball installer, which comes with several skins and extensions. You can copy and paste the skins/ directory from it.