Laravel 5.4: Installation failed, reverting ./composer.json and ./composer.lock to their original content while trying to install a package [duplicate]

I'm working with Laravel 5.4 and I wanted to install Laravel Sluggable package but I got this error on the Terminal:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - cviebrock/eloquent-sluggable[8.0.0, ..., 8.0.8] require illuminate/config ^8.0 -> found illuminate/config[v8.0.0, ..., v8.77.1] but these were not loaded, li
kely because it conflicts with another require.
    - Root composer.json requires cviebrock/eloquent-sluggable ^8.0 -> satisfiable by cviebrock/eloquent-sluggable[8.0.0, ..., 8.0.8].

Installation failed, reverting ./composer.json and ./composer.lock to their original content.

And here is my Composer.json file:

{
    "name": "laravel/laravel",
    "description": "The Laravel Framework.",
    "keywords": ["framework", "laravel"],
    "license": "MIT",
    "type": "project",
    "require": {
        "php": ">=5.6.4",
        "laravel/framework": "5.4.*",
        "laravel/tinker": "~1.0"
    },
    "require-dev": {
        "fzaninotto/faker": "~1.4",
        "mockery/mockery": "0.9.*",
        "phpunit/phpunit": "~5.7"
    },
    "autoload": {
        "classmap": [
            "database"
        ],
        "psr-4": {
            "App\\": "app/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\": "tests/"
        }
    },
    "scripts": {
        "post-root-package-install": [
            "php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ],
        "post-create-project-cmd": [
            "php artisan key:generate"
        ],
        "post-install-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postInstall",
            "php artisan optimize"
        ],
        "post-update-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postUpdate",
            "php artisan optimize"
        ]
    },
    "config": {
        "preferred-install": "dist",
        "sort-packages": true,
        "optimize-autoloader": true
    }
}

I don't know really what's going wrong here and how can I fix this issue.. So if you know please let me know, cause I really need it.

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