Problem with delivery customization with GraphiQL

[Alex-bml]

I'm trying to get started with building delivery customizations like this https://shopify.dev/apps/checkout/delivery-customizations/getting-started

Got to Step 2: Create the delivery customization with GraphiQL

mutation {
  deliveryCustomizationCreate(deliveryCustomization: {
     functionId: "01GR3T9F6MN4NSQXCHJWN769JA"
     title: "Add message to delivery options for state/province"
     enabled: true
}) {
  deliveryCustomization {
     id
}
  userErrors {
    message
}
}
}

I get an response:

{
  "data": {
    "deliveryCustomizationCreate": null
  },
  "errors": [
  {
     "message": "Delivery Customization API is not enabled for the current shop.",
.........

I did check to see that I'm using the Checkout Extensibility Preview Store which is required, so that's not the issue.

I did Check granted access scopes:

{
  "data": {
    "appInstallation": {
      "accessScopes": [
....
      {
        "handle": "write_delivery_customizations",
        "description": "Write access for delivery customizations"
       },
......
      {
        "handle": "read_delivery_customizations",
        "description": "Read access for delivery customizations"
      }
   ]
}
}

What is my mistake?

Thanks