Kubernetes secrets are only accessible in the namespace they're created in. A quick one-liner to copy a secret from namespaceA to namespaceB is:

kubectl get secrets <secret_name> --export --namespace=<namespaceA> -o yaml | kubectl apply --namespace=<namespaceB> -f -

An example I use to copy my Let's Encrypt cert from the default namespace to another namespace:

kubectl get secrets wc-k8s-prod-tls --export --namespace=default -o yaml | kubectl apply --namespace=monitoring -f -

Note: --export has been deprecated, but it is a simple way to remove some of the auto-generated keys like resourceVersion, uid, selfLink, etc.