openstack-heat and version

Question

I have this hot template. I keep getting this error; Cannot convert datetime.date(2021, 4, 16) to primitive.

the code is muc longer than this, but looks very similar, just with more servers and each server has its own flouating ip and port.

heat_template_version: 2021-04-16


description: >
  HOT template to create a new neutron network plus a router to the public
  network, and for deploying two servers into the new network. The template also
  assigns floating IP addresses to each server so they are routable from the
  public network.

parameters:
  key_name:
    type: string
    description: Name of keypair to assign to servers
    default: darwin
  image:
    type: string
    description: Name of image to use for servers
    default: db1bc18e-81e3-477e-9067-eecaa459ec33 # ubuntu22.04
  flavor:
    type: string
    description: Flavor to use for servers
    default: gx1.2c4r
  public_net:
    type: string
    description: >
      ID or name of public network for which floating IP addresses will be allocated
    default: nx-internal
  private_net_name:
    type: string
    description: Name of private network to be created
    default: ntnu-internal
  private_net_cidr:
    type: string
    description: Private network address (CIDR notation)
    default: 172.168.100.0/27
  private_net_gateway:
    type: string
    description: Private network gateway address
    default: 172.168.100.1
  # private_net_pool_start:
  #   type: string
  #   description: Start of private network IP address allocation pool
  # private_net_pool_end:
  #   type: string
  #   description: End of private network IP address allocation pool

resources:
  private_net:
    type: OS::Neutron::Net
    properties:
      name: { get_param: private_net_name }

  private_subnet:
    type: OS::Neutron::Subnet
    properties:
      network_id: { get_resource: private_net }
      cidr: { get_param: private_net_cidr }
      gateway_ip: { get_param: private_net_gateway }
      # allocation_pools:
      #   - start: { get_param: private_net_pool_start }
      #     end: { get_param: private_net_pool_end }

  router:
    type: OS::Neutron::Router
    properties:
      external_gateway_info:
        network: { get_param: public_net }

  router_interface:
    type: OS::Neutron::RouterInterface
    properties:
      router_id: { get_resource: router }
      subnet_id: { get_resource: private_subnet }

  Ranger:
    type: OS::Nova::Server
    properties:
      name: Ranger
      image: { get_param: image }
      flavor: { get_param: flavor }
      key_name: { get_param: key_name }
      networks:
        - port: { get_resource: Ranger_port }


  Ranger_port:
    type: OS::Neutron::Port
    properties:
      network_id: { get_resource: private_net }
      fixed_ips:
        - subnet_id: { get_resource: private_subnet }

  Ranger_floating_ip:
    type: OS::Neutron::FloatingIP
    properties:
      floating_network: { get_param: public_net }
      port_id: { get_resource: Ranger_port }

outputs:
  Ranger_private_ip:
    description: IP address of Ranger in private network
    value: { get_attr: [ Ranger, first_address ] }

tried differeny yaml lint tools no erroer from there. i have tried deploying onse server each time ie shortening the template. still the same error though