There are three possible solutions.

1. Specify namespace in the kubectl command:

kubectl apply -f https://k8s.io/examples/controllers/nginx-deployment.yaml -n my-namespace

2. Specify namespace in your yaml files:

  apiVersion: apps/v1
  kind: Deployment
  metadata:
    name: my-deployment
    namespace: my-namespace

3. Change default namespace in ~/.kube/config:

apiVersion: v1
kind: Config
clusters:
- name: "k8s-dev-cluster-01"
  cluster:
    server: "https://example.com/k8s/clusters/abc"
    namespace: "my-namespace"

First you need to create the namespace likes this

kubectl create ns nameOfYourNamespace

Then you create your deployment under your namespace

kubectl apply -f https://k8s.io/examples/controllers/nginx-deployment.yaml -n examplenamespace

The ns at

kubectl create ns nameOfYourNamespace

stands for namespace

The -n

kubectl apply -f https://k8s.io/examples/controllers/nginx-deployment.yaml -n examplenamespace

stands for --namespace

So you first create your namespace in order Kubernetes know what namespaces dealing with.

Then when you are about to apply your changes you add the -n flag that stands for --namespace so Kubernetes know under what namespace will deploy/ create the proper resources