After you run the kubeadm init command in your master node, it will show you that you need to run the below-given command to your master node.
mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config
If you get the same error again, then check the context in your cluster using the below command.
$ kubectl config view
If the context is not set, then you need to set that.