I can't install spatie/laravel-sitemap on Laravel 5.6

I'm getting this error when installing:

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

Problem 1 - spatie/laravel-sitemap[5.9.0, ..., 5.9.2] require illuminate/support ^8.0 -> found illuminate/support[v8.0.0, ..., 8.x-dev] but these were not loaded, likely because it conflicts with another require. - Root composer.json requires spatie/laravel-sitemap ^5.9 -> satisfiable by spatie/laravel-sitemap[5.9.0, 5.9.1, 5.9.2].

You can also try re-running composer require with an explicit version constraint, e.g. "composer require spatie/laravel-sitemap:*" to figure out if any version is installable, or "composer require spatie/laravel-sitemap:^2.1" if you know which you need.

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

via Chebli Mohamed