Is it possible to register a Namespace with out giving a credit card? It seems it is not - I end up in the step 2, but I am not able to define namespaces and do any management.
Remarkbox is pay-what-you-can and does not require a credit card.
Often when people experience this issue it is because Remarkbox is not able to verify that you own the domain. This can happen when there is a mismatch between the domain used to register the Namespace and the domain that your site is hosted on.
Another common issue is that Remarkbox could not reach your site or if it can, it was not able to parse the Namespace owner key (UUID).
If you have a test domain please register that domain as it's own Namespace separately.
I hope this helps!