The bundle available for MW 1.30 via the ExtensionDistributor, i.e REL1_30-c75b9d4, is missing the file "extension.json", causing installation through wfLoadExtension( 'Variables' );
to fail.
Topic on Extension talk:Variables
The docu states: "To users running MediaWiki 1.30 or earlier", so for MW 1.30 this extension has to be invoked via require_once ...
.