Resource [parameters('mgName')] Location must be an expression or 'global'

Question

I am experimenting with Azure Management Groups Arm template.

As you can see in this link, I have this Arm template:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-08-01/managementGroupDeploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "mgName": {
      "type": "string",
      "defaultValue": "[concat('mg-', uniqueString(newGuid()))]"
    }
  },
  "resources": [
    {
      "type": "Microsoft.Management/managementGroups",
      "apiVersion": "2021-04-01",
      "name": "[parameters('mgName')]",
      "scope": "/",
      "location": "eastus",
      "properties": {}
    }
  ],
  "outputs": {
    "output": {
      "type": "string",
      "value": "[parameters('mgName')]"
    }
  }
}

Saved as mg.json and it works fine.

Later I start experimenting with validating and testing Arm template using Test-AzTemplate (https://github.com/Azure/arm-ttk). When I run following command to test Arm Template:

Test-AzTemplate -TemplatePath .\mg.json

I get this test error:

[-] Resources Should Have Location (3 ms)
    Resource [parameters('mgName')] Location must be an expression or 'global'

Now when I remove "location": "eastus", line form Arm template, the test does not fail and pass the test.

My Question:

Is this location in Management Group Arm required or not required? And why it is failing when it is part of Microsoft documentation! Any idea?