Whoops! The default skin for your wiki, defined in $wgDefaultSkin as minerva, 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.

minervaneue / MinervaNeue (disabled)
monobook / MonoBook (disabled)
timeless / Timeless (disabled)
vector / Vector (enabled)

You can paste the following lines into LocalSettings.php to enable all installed skins:

wfLoadSkin( 'MinervaNeue' );
wfLoadSkin( 'MonoBook' );
wfLoadSkin( 'Timeless' );

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.
Downloading individual skin tarballs from mediawiki.org.
Using Git to download skins.

Doing this should not interfere with your Git repository if you're a MediaWiki developer.