Your requirements could not be resolved to an installable set of packages.
Problem 1
- shinsenter/defer-laravel 1.0.4 requires shinsenter/defer.php ^2.4 -> satisfiable by shinsenter/defer.php[2.4.0, 2.4.1, 2.4.2].
- shinsenter/defer-laravel 1.0.3 requires shinsenter/defer.php ^2.3 -> satisfiable by shinsenter/defer.php[2.3.0, 2.4.0, 2.4.1, 2.4.2].
- shinsenter/defer-laravel 1.0.2 requires shinsenter/defer.php ^2.2 -> satisfiable by shinsenter/defer.php[2.2.0, ..., 2.4.2].
- shinsenter/defer.php[2.2.0, ..., 2.4.2] require symfony/css-selector ^2.0 || ^3.0 || ^4.0 || ^5.0 -> found symfony/css-selector[2.0.4, ..., 2.8.x-dev, v3.0.0-BETA1, ..., 3.4.x-dev, v4.0.0-BETA1, ..., 4.4.x-dev, v5.0.0-BETA1, ..., 5.4.x-dev] but it conflicts with your root composer.json require (^6.0).
- Root composer.json requires shinsenter/defer-laravel ^1.0 -> satisfiable by shinsenter/defer-laravel[1.0.2, 1.0.3, 1.0.4].
Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.
You can also try re-running composer require with an explicit version constraint, e.g. "composer require shinsenter/defer-laravel:*" to figure out if any version is installable, or "composer require shinsenter/defer-laravel:^2.1" if you know which you need.