Json for the rule. The tag could be something else. Just change the field: “tags.Costcenter”

{
  "mode": "All",
  "policyRule": {
    "if": {
      "not": {
        "field": "tags.Costcenter",
        "in": "[parameters('allowedTagValues')]"
      }
    },
    "then": {
      "effect": "deny"
    }
  },
  "parameters": {
    "allowedTagValues": {
      "type": "Array",
      "metadata": {
        "displayName": "Allowed tag values",
        "description": "The list of allowed Costcenter tag values"
      }
    }
  }
}

Parameters when assigning the policy

[
  "IT",
  "Risk",
  "Advisory",
  "Legal"
]