//validate schema
var jsonData = pm.response.json();
var loginSchema = {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "type": "object",
    "required": [
        "id", "token", "userName", "password", "token"
    ],
    "properties": {
        "id": {"type": "number"},
        "firstName": {"type": "string"},
        "lastName": {"type": "string"},
        "userName": {"type": "string"},
        "password": {"type": ["string", "null"]},
        "token": {"type": "string"}
    }
};

var validationResults = tv4.validateMultiple(jsonData, loginSchema);
console.log(validationResults.errors);