有用的解决方法：

实际上，这个问题是因为没有给集群的pod指定内网网段的缘故。最彻底的解决办法是将这个集群所有节点重新kubeadm reset，重新初始化，并在初始化时指定内网网段：

kubeadm init --image-repository=registry.aliyuncs.com/google_containers --pod-network-cidr=10.244.0.0/16

解决k8s配置时不能打开/run/flannel/subnet.env的问题 - 简书

之后kubectl create -f nginx-pod.yaml时

之后又出现了：

Unable to connect to the server: tls: failed to verify certificate: x509: certificate signed by unknown authority (possibly because of "crypto/rsa: verification error" while trying to verify candidate authority certificate "kubernetes"

解决方法：

mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config

Unable to connect to the server x509 certificate signed by unknown authority possibly because of crypto rsa verification error | Edureka Community