Thread:Extension talk:UserMerge/Fatal error: Call to undefined method User::getEditToken()/reply

You use an outdated MediaWiki Version 1.18.x, which is no longer supported, see Download. Since version 1.19, the old method editToken has been deprecated, and the UserMerge extension uses the new name getEditToken. And you haven't included the version number of the extension - this should be 1.7.2.

Ad-hoc fix for pre 1.19 MediaWiki versions: it should work if you rename the call in UserMerge_body.php from User::getEditToken to User::editToken.