Whoops! The default skin for your wiki, defined in $wgDefaultSkin as monobook, is not available.
Your installation seems to include the following skins. See Manual: Skin configuration for information how to enable them and choose the default.
cologneblue / CologneBlue (disabled)
modern / Modern (disabled)
monobook / MonoBook (disabled)
vector / Vector (disabled)
If you have just installed MediaWiki
You probably installed from git, or directly from the source code using some other method. This is expected. 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.
Cloning one of the mediawiki/skins/* repositories via git into the skins/ directory of your MediaWiki installation.
Doing this should not interfere with your git repository if you're a MediaWiki developer.
If you have just upgraded MediaWiki
MediaWiki 1.24 and newer no longer automatically enables installed skins (see Manual: Skin autodiscovery). You can paste the following lines into LocalSettings.php to enable all currently installed skins: