This is because you most likely don't have the requesthandler configured in on the platform portal for this app. There's a tab on your environment details page that allows you to add these links.
Note that this is only possible for full cloud environments, you won't be able to publish rest webservices in a sandbox.
I have set up the rest handler for my production environment, but when I try to call my .../rest overview it says: "301 Moved permanently"
Am I missing something?
In addition to Achiel's answer;
Recent versions of the REST services modules can run in a 'sandbox compatibility' mode, but then the endpoint of your services are not at /rest/ at /ws-doc/. (Meaning your wsdl documentation will no longer be remotely visible).
This is a temporarily workaround for sandbox environments which is applied automatically if you didn't specify a specific endpoint yourself. Please note that it is not guaranteed that this will be working in the future, but it might help you out for simple PoC's.