Uhm, strange. Can you try to also open a pyspark or a scala shell (with spark-client.pyspark or spark-client.spark-shell) using the same spark service account? Can you also make sure the resources indeed exists by using kubectl get sa -n demonamespace and kubectl get secrets -n demonamespace?
Also just one tip: maybe you should store all those configuration in the service account using:
spark-client.service-account-registry add-config (see here) such that you don’t have to type that all the times