The error means that there are multiple networks available and you need to specify the network-id which you want your instance to be on. On nova CLI, it is specified as '--nic net-id=<network-id>' where network-id is the ID returned by 'nova network list' command. AFAIU, add this parameter to your template and give it a try!</network-id>

This happens when you are using Neutron and have multiple Networks defined (if only one Network exists, the server can be attached to it by default, otherwise it is ambiguous). The solution is to specify a specific Network to attach to. The way to do this depends on the resource type.

For AWS::AutoScaling::AutoScalingGroup resources, add the VPCZoneIdentifier property to the scaling group (not launch configuration) resource, with the value being the UUID of a Neutron Subnet.

For AWS::EC2::Instance resources, the easiest way is to add the property SubnetId to the resource, with the value being the UUID of a Neutron Subnet.

You can pass the Subnet UUID in as a parameter to the template, either through the environment or on the command line, or reference a Subnet created in the template using the OS::Neutron::Subnet resource (note that this requires a Network UUID as a parameter, and the same applies - you can pass it in or create it in the template with an OS::Neutron::Network resource).

If you want to keep your template compatible with CloudFormation, the AWS::EC2::VPC and AWS::EC2::Subnet resources in Heat also return the UUID of a Neutron Network and Subnet, respectively, when they are referenced in a template.

For OS::Nova::Server resources, the syntax is a little more complicated (and powerful), but it looks like this:

resources:
  my_server:
    type: OS::Nova::Server
    properties:
        networks:
          - network: <Network name or UUID>

Just replace <Network name or UUID> with the Network name or UUID. Again, you can pass it in as a parameter, or reference an OS::Neutron::Network resource in the template.

As the message indicates, you need to specify a network ID. Unfortunately, from the Heat documentation it is not obvious how to specify network ID in the CFN template. After looking at the Heat source code and some trial and errors, I was able to figure it out. Here's what you have to do:

1. Add an "SubnetID" section in the template (see example below).

2. Specify the subnetId as instance properties. You can get the subnetId either from Horizon dashboard or using neutron net-list command

My Heat template (only showing partial file):

{
  "AWSTemplateFormatVersion" : "2010-09-09",

  "Description" : "AWS CloudFormation Sample Template WordPress_Single_Instance: WordPress is web software you can use to create a beautiful website or blog. This template installs a single-instance WordPress deployment using a local MySQL database to store the data.",

      "Parameters" : {

        "KeyName" : {
          "Description" : "Name of an existing EC2 KeyPair to enable SSH access to the instances",
          "Type" : "String"
        },
        "SubnetId" : {
          "Description" : "Name of an existing EC2 KeyPair to enable SSH access to the instances",
          "Type" : "String"
        },

And Properties file, named aws-environ.yaml:

parameters:
  InstanceType: m1.medium 
  KeyName: imtiaz-key
  SubnetId: 06edc29e-0103-469a-ad34-08c6e7994016

To launch an instance, use:

heat stack-create -f /etc/heat/templates/Wordpress_Single_Instance.template -e /etc/heat/templates/aws-environ.yaml my-stack