Comment 2 for bug 1337260

It seems you are creating an instance, and the network is missing when create the instance.

The following is an example of Instance:
heat_template_version: 2015-04-30
...
resources:
  ...
  the_resource:
    type: OS::Nova::Server
    properties:
      admin_pass: String
      availability_zone: String
      block_device_mapping: [{"snapshot_id": String, "volume_id": String, "volume_size": Integer, "device_name": String, "delete_on_termination": Boolean}, {"snapshot_id": String, "volume_id": String, "volume_size": Integer, "device_name": String, "delete_on_termination": Boolean}, ...]
      block_device_mapping_v2: [{"device_name": String, "swap_size": Integer, "disk_bus": String, "boot_index": Integer, "snapshot_id": String, "image_id": String, "volume_size": Integer, "volume_id": String, "delete_on_termination": Boolean, "device_type": String}, {"device_name": String, "swap_size": Integer, "disk_bus": String, "boot_index": Integer, "snapshot_id": String, "image_id": String, "volume_size": Integer, "volume_id": String, "delete_on_termination": Boolean, "device_type": String}, ...]
      config_drive: Boolean
      diskConfig: String
      flavor: String
      flavor_update_policy: String
      image: String
      image_update_policy: String
      key_name: String
      metadata: {...}
      name: String
      networks: [{"subnet": String, "uuid": String, "network": String, "port": String, "port_extra_properties": {"allowed_address_pairs": [{"mac_address": String, "ip_address": String}, {"mac_address": String, "ip_address": String}, ...], "admin_state_up": Boolean, "qos_policy": String, "binding:vnic_type": String, "value_specs": {...}, "port_security_enabled": Boolean, "mac_address": String}, "fixed_ip": String}, {"subnet": String, "uuid": String, "network": String, "port": String, "port_extra_properties": {"allowed_address_pairs": [{"mac_address": String, "ip_address": String}, {"mac_address": String, "ip_address": String}, ...], "admin_state_up": Boolean, "qos_policy": String, "binding:vnic_type": String, "value_specs": {...}, "port_security_enabled": Boolean, "mac_address": String}, "fixed_ip": String}, ...]
      personality: {...}
      reservation_id: String
      scheduler_hints: {...}
      security_groups: [Value, Value, ...]
      software_config_transport: String
      user_data: String
      user_data_format: String

And the definition of network is as below, you need to set an available network name or uuid for the instance.

network

Name or ID of network to create a port on.

String value expected.

Can be updated without replacement.

Value must be of type neutron.network