Thread:Extension talk:Scribunto/Fatal error: Class 'MapCacheLRU' not found in /var/www/mediawiki/extensions/Scribunto/engines/LuaCommon/UstringLibrary.php on line 37/reply

You're probably using a too-new version of Scribunto with a too-old version of MediaWiki itself. You'd either want to use the branch of Scribunto corresponding to your version of MediaWiki, or upgrade your MediaWiki to match the version of Scribunto you're trying to use.