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']);

Thanks! I solved the problem using this

Events.php

PHP Code:

<?php namespace Config;

use CodeIgniter\Events\Events;
use App\Eventos\Compress;

$Compress = new Compress();

Events::on('pre_system', [$Compress, 'Compression']); 


Compress.php

PHP Code:

<?php namespace App\Eventos;

class Compress {
public function Compression(){

echo "test";

  }
} 


Anyway, thank you very much!