How can I have data persistence on my cluster?

Kapsule is a managed Kubernetes engine. By definition, the nodes of your clusters can be deleted, replaced, or restarted if the applications running on it require it or if a node suddenly stops responding. It means that Kubernetes clusters are to be considered stateless.
If you require a stateful application, you can use Persistent Volumes.
The storageClass for Scaleway Block Storage volumes is set by default, so it does not need to be specified.