Fatal error in WordPress 4.8

Hello, when i try to activate the 1.3.9 version in WordPress, i have a Fatal Error :

Warning: require_once(__DIR__/composer/autoload_real.php) [function.require-once]: failed to open stream: No such file or directory in /public_html/wp-content/plugins/mivhak/vendor/autoload.php on line 5

Fatal error: require_once() [function.require]: Failed opening required ‘__DIR__/composer/autoload_real.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /public_html/wp-content/plugins/mivhak/vendor/autoload.php on line 5

I have no idea what to do. Could you help me please ?

Thx for France.