CI 2.2.6 subfolder route 403 error

You need to remove the fysical admin folder, or ajust your .htaccess file. By default it is configured to redirect all requests to non existing files or folders to index.php.
Since the folder exists in your structure you don't access your admin controller. You can a) tweak your .htaccess into ignoring the existense of an admin folder or b) change your folder structure to something like: /assets/admin/css|js|fonts for example.