NFS storage is tricky in itself and with k8s, so setting it up for the user pods also becomes a bit tricky.
I think you have created a volume, and your Helm chart configuration is almost correct (one space to little before claimName) in order to mount a volume. You should be able to confirm this by:
# ensure you find a volume on pod
# ensure you find a volumeMount on pod's container
kubectl get pod jupyter-myuserpod -o yaml
If you get this working, I think you may still run into issues of the NFS server providing files that the typical jovyan user with user id of 1000 won’t be allowed to access or similar, which can force you to use a “volume mount hack” as well.
I recall this thread was very useful for me when I set up NFS storage initially: https://github.com/jupyterhub/zero-to-jupyterhub-k8s/issues/421