See https://laravel.com/docs/5.4/upgrade

>set() method should be changed to ->put()

....

Sessions:Symfony Compatibility

Laravel's session handlers no longer implements Symfony's SessionInterface. Implementing this interface required us to implement extraneous features that were not needed by the framework. Instead, a new Illuminate\Contracts\Session\Session interface has been defined and may be used instead. The following code changes should also be applied:

All calls to the ->set() method should be changed to ->put(). Typically, Laravel applications would never call the set method since it has never been documented within the Laravel documentation. However, it is included here out of caution.

All calls to the ->getToken() method should be changed to ->token().

All calls to the $request->setSession() method should be changed to setLaravelSession().