I think you have RBAC activatet inside your Cluster. You need to assign a ServiceAccount to your pod which containing a Role, that allows this ServerAccount to get a list of all Namespaces. When no ServiceAccount is specified in the Pod-Template, the namespaces default ServiceAccount will be assigned to the pods running in this namespace.

First, you should create the Role

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  namespace: <YOUR NAMESPACE>
  name: namespace-reader
rules:
- apiGroups: [""] # "" indicates the core API group
  resources: ["namespaces"] # Resource is namespaces
  verbs: ["get", "list"] # Allowing this roll to get and list namespaces

Create a new ServiceAccount inside your Namespace

apiVersion: v1
kind: ServiceAccount
metadata:
  name: application-sa
  namespace: <YOUR-NAMESPACE>

Assign your Role created Role to the Service-Account:

apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: allow-namespace-listing
  namespace: <YOUR-NAMESPACE>
subjects:
- kind: ServiceAccount
  name: application-sa # Your newly created Service-Account
  namespace: <YOUR-NAMESPACE>
roleRef:
  kind: Role
  name: namespace-reader # Your newly created Role
  apiGroup: rbac.authorization.k8s.io

Assign the new Role to your Pod by adding a ServiceAccount to your Pod Spec:

apiVersion: v1
kind: Pod
metadata:
  name: podname
  namespace: <YOUR-NAMESPACE>
spec:
  serviceAccountName: application-sa

You can read more about RBAC in the official docs. Maybe you want to use kubectl-Commands instead of YAML definitions.