Use ARM to create app-service in resource group

Question

Someone in my company set up a resource group called Customers in our Azure portal. I have a successful build pipeline and now I want to create a release pipeline.

If I create a App Service in the Azure Portal and link it to the Customers resource group I can successfully release my React application. But I want to automate this process.

I've created a ARM template:

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {},
    "variables": {},
    "resources": [
        {
            "type": "Microsoft.Web/serverfarms",
            "apiVersion": "2016-09-01",
            "name": "movieseatcustomers",
            "location": "[resourceGroup().location]",
            "sku": {
                "name": "F1",
                "capacity": 1
            },
            "tags": {
                "displayName": "movieseatcustomers"
            },
            "properties": {
                "name": "movieseatcustomers"
            }
        },
        {
            "type": "Microsoft.Web/sites",
            "apiVersion": "2018-02-01",
            "name": "movieseatcustomers1234",
            "location": "[resourceGroup().location]",
            "tags": {
                "[concat('hidden-related:', resourceGroup().id, '/providers/Microsoft.Web/serverfarms/movieseatcustomers')]": "Resource",
                "displayName": "movieseatcustomers1234"
            },
            "dependsOn": [
                "Microsoft.Web/serverfarms/movieseatcustomers"
            ],
            "properties": {
                "name": "movieseatcustomers1234",
                "serverFarmId": "[resourceId('Microsoft.Web/serverfarms/', 'movieseatcustomers')]"
            }
        }
    ],
    "outputs": {}
}

But in my release pipeline I get the error:

2019-05-31T11:27:11.0415849Z ##[error]BadRequest: {
"Code": "BadRequest",
    "Message": "Requested feature is not available in resource group Customers. Please try using a different resource group or create a new one.",

What's the deal? Am I implementing the creating of a app service wrongly? Do I need special rights to do it through a ARM template?