Validation Rules Group Error

This is my Validation.php

PHP Code:

//--------------------------------------------------------------------
 // Rules
//--------------------------------------------------------------------

public $login = [
        'username'    => 'required|valid_email',
        'password'    => 'required'
    ];

    public $login_errors = [
        'username' => [
            'required'    => 'Sorry, those credentials are incorrect',
            'valid_email' => 'Sorry, those credentials are incorrect'
        ],
        'password'    => [
            'required' => 'Sorry, those credentials are incorrect'
        ]
    ];

    public $reset_password = [
        'username'    => 'required|valid_email'
    ];

    public $reset_password_errors = [
        'username' => [
            'required'    => 'Please enter a valid email address',
            'valid_email' => 'Please enter a valid email address'
        ]
    ];

    public $update_password = [
        'password'          => 'required|min_length[10]',
        'password_again'    => 'matches[password]'
    ];

    public $update_password_errors = [
        'password' => [
            'required'    => 'Passwords must be at least 10 characters',
            'min_length'  => 'Passwords must be at least 10 characters'
        ],
        'password_again' => [
            'matches[password]' => 'Your passwords do not match'
        ]
    ];