I created the module

schema.graphqls

#Magento Customer GraphQl Schema

type Query {
    testcustomer: Testcustomer @resolver(class: "Graphql\CrudOperations\Model\Resolver\StudentList") @doc(description: "The testcustomer query returns information about a customer")
}

type Testcustomer @doc(description: "Testcustomer defines the customer name and other details") {

student_id: Int 
student_name: String 
dept: String 
subject: String 

}

Postman

query testcustomer {
    student_id
    student_name
    dept
    subject
}

But throwing error like below

Please anyone help.