Unable to create new site with ARM template (bicep)

```
var customDomains = [
'test.com'
'www.test.com'
]

resource site 'Microsoft.Web/staticSites@2021-01-15' = {
location: resourceGroup().location
name: 'site'
properties: {
buildProperties:{
skipGithubActionWorkflowGeneration: true
}
allowConfigFileUpdates: true
}
sku: {
name: 'Free'
tier: 'Free'
}
identity: {
type: 'SystemAssigned'
}
resource domains 'customDomains' = [for domain in customDomains: {
name: domain
properties: {
validationMethod: 'dns-txt-token'
}
}]
}

```

When executing this via `az deployment group create` I get an error.

```
{
"code": "DeploymentFailed",
"message": "At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/DeployOperations for usage details.",
"details": [
{
"message": "SkuCode 'SKU' is invalid."
}]
}
```

If I remove the identity however, it goes through correctly.

I would expect that given the free tier can have managed functions, I should be able to assign a system managed identity in the template. Or alternatively, add it to the SKU documentation in the template reference that I can't.