Manual:Mctest.php

Details
mctest.php is a MediaWiki maintenance script that makes several 'set', 'incr', 'get' requests on every memcached server and shows a report.

Common error
This error comes when you MediaWiki installation is not configured properly with memcached server.
 * MediaWiki isn't configured for Memcached usage

This means, you have not configured the '$cache' properly in $wgObjectCaches. Check LocalSettings.php for configuration.
 * MediaWiki isn't configured with a cache named '$cache'