MountVolume.SetUp failed for volume "mongo" : hostPath type check failed: /mongo/data is not a directory
If by any chance you are using minikube, then don't forget that minikube itself is a container. So hostPath points to a path in that container, not to a path on your host machine. You have to mount your PC path into the minikube container and then into the POD.
Example:minikube start --mount --mount-string="/host/path:/minikubeContainer/path"
Removing type: Directory from the mongodb.yml works as expected
Update:
In new versions, changing type: Directory by type: DirectoryOrCreate, creates the directory.