Packer-centos7 basition key error

centos7.json

{
  "variables": {
   "vsphere_uid": "Null",
   "vsphere_passwd": "Null",
   "vm_superuser_passwd" : "Null",
   "vsphere_vm_name": "Null",
   "bastion_host": "",
   "bastion_user": "",
   "bastion_key":"Null",
   "datacenter": "",
   "cluster": "",
   "datastore": "",
   "network": "",
   "iso_paths": ""
   },

  "builders": [
    {
      "type": "vsphere-iso",

      "vcenter_server":      "{{user `vsphere_server` }}",
      "username":            "{{user `vsphere_uid` }}",
      "password":            "{{user `vsphere_passwd` }}",
      "insecure_connection": "true",
      "datacenter": "{{user `datacenter` }}",
      "cluster": "{{user `cluster` }}",
      "datastore": "{{user `datastore` }}",
      "network": "{{user `network` }}",
      "vm_name": "{{user `vsphere_vm_name` }}",

      "guest_os_type": "centos7_64Guest",

      "ssh_username": "nubi-admin",
      "ssh_password": "{{user `vm_superuser_passwd` }}",
      "ssh_bastion_host": "{{user `bastion_host` }}",
      "ssh_bastion_username": "{{user `bastion_user` }}",
      "ssh_bastion_private_key_file": "{{user `bastion_key` }}",
      "ssh_bastion_agent_auth": true,
     
      "CPUs":             2,
      "RAM":              5120,
      "RAM_reserve_all": false,

      "convert_to_template": true,

      "disk_controller_type":  "pvscsi",
      "disk_size":        102400,
      "disk_thin_provisioned": true,
      "network_card": "vmxnet3",

      "iso_paths": [
        "{{user `iso_paths` }}"
      ],
      "iso_checksum": "13675c6f74880e7ff3481b91bdaf925ce81bda8f",
      "iso_checksum_type": "sha1",
      "floppy_files": [
        "{{template_dir}}/ks.cfg"
      ],
      "boot_command": "<esc> linux inst.text inst.ks=hd:fd0:/ks.cfg <enter><wait>"
          }
  ]


}