Unknown configuration key: '"virtual_machine"'

Hi, I would like to use Packer to create LxD vm image. Here is my json looks like:

  "builders": [
      "type": "lxd",
      "virtual_machine": true,
      "name": "lxd-focal-vm",
      "image": "images:ubuntu/focal/cloud",
      "output_image": "ubuntu-focal-vm",
      "publish_properties": {
        "description": "Trivial repackage with Packer"

when packer validate I got the following error though:

Error: Failed to prepare build: "lxd-focal-vm"

1 error occurred:
	* unknown configuration key: '"virtual_machine"'

Even I use packer fix, it still remains the same.

My packer version v1.6.1 on Ubuntu 20.04 LTS.

May someone help?