by Richard Oelmann.
From your screenshot this plugin is dated from around 2012 which probably makes it outdated and not suitable for later versions of Moodle2
The error tells you what is wrong, but that is only one symptom of a plugin which is not suitable for your version of Moodle
https://moodle.org/plugins/pluginversions.php?plugin=assignment_random - developed upto Moodle2.2, not beyond.