How do I redirect users after they submit a form?
You're getting that error because fromRoute() takes the route name as the argument, not a literal path.

Assuming you know the node id you want to redirect to:

$url = Url::fromRoute('entity.node.canonical', array('node' =&gt; $node_id));


You don't ever want to redirect to internal paths like that (in your question) because they are subject to change.