Using Event file

You need to pass a valid callback: https://www.php.net/manual/en/language.t...llable.php
If you're going to use the array format it needs to be [$object, $method] where object is an instance or a class name. For example if you have "app/Libraries/Compress.php" you could use something like:
Events::on('pre_system', ['App\Libraries\Compress', 'compresstest']);