Issue while Querying GraphQL API as a client

I am trying to run a query from postman like below and see HTTP 400 bad request as the error. Please help.

query getContract($data : String!){
  contract(contractNumber: $data) {
    contractNumber
    versions(getCurrentContractVersion: true) {
      nodes {
        customer {
          birthDate
          legalEntityCode
        }
        contractAssets {
          nodes {
            assetDetails {
              plate
            }
          }
        }
      }
    }
  }
}

Postman Response - HTTP 400 Bad request

{
    "errors": [
        {
            "message": "Either the parameter query or the parameter id has to be set.",
            "extensions": {
                "code": "HC0013"
            }
        }
    ]
}