Hi.
Check that the api server is actually running and hasn’t crashed:
docker ps | grep kube-apiserver
But the most likely problem and the one that usually gets me is that you don’t have a .kube directory with the right config in it. Try this:
mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config