Adobe Photoshop API - Firefly Services

I am trying to user Adobe Photoshop API to create PSD with groups, i.e. https://developer.adobe.com/firefly-services/docs/photoshop/api/photoshop-v2/, to create a PSD file programmatically. I tried to create a simple PSD with groups of layers. But even though I think I followed the API schema, I still get “An unknown error occurred” error response.

For example a payload like below trying to create a PSD with one group that has 2 image layers in it. 

{
    "image": { "source": { "url": "<GET_BACKGROUND>" } },
    "edits": { "layers": [
      {
        "name": "group1", "type": "group_layer",
        "operation": { "type": "add", "placement": { "type": "top" } },
        "children": [
          { "name": "source", "type": "layer",
            "operation": { "type": "add", "placement": { "type": "top" } },
            "image": { "source": { "url": "<Image1>" } } },
          { "name": "add", "type": "layer",
            "operation": { "type": "add", "placement": { "type": "top" } },
            "image": { "source": { "url": "<Image2>" } },
            "blendOptions": { "blendMode": "multiply" } }
        ]
      }
    ] },
    "outputs": [ { "mediaType": "image/vnd.adobe.photoshop", "destination": { "url": "<PUT>" } } ]
  }

Anyone has tried this and has any suggestions? Thanks!