You can also do like this if you're only interested in extracting the "user_id":

String userId = 
given().
        contentType("application/json").
        body(requestBody).
when().
        post("/admin").
then().
        statusCode(200).
extract().
        path("user_id");

In its simplest form it looks like this:

String userId = get("/person").path("person.userId");

There are two things that we need to rectify.

First, there is a typo in the construction of the json-object:

requestParams.put("client_secrets", "Gsv7brPlt5fIpqa1ebFc5A1uHwDJBPY6TRaMm8wa");

should be:

requestParams.put("client_secret", "Gsv7brPlt5fIpqa1ebFc5A1uHwDJBPY6TRaMm8wa");

(plural vs singular)

Next, when we make the actual request, we need to base the request on the requestParams we constructed. This line:

response = RestAssured.post();

should therefore be changed to:

response = requestSpecification.post();

With those changes, we can run the testGetAllLanguages-test and it will succeed.

You can find a patch with the changes listed above here (gist.github.com).

I suggest refactoring the code and make use of RestAssure's fluent API. A refactoring may look like this:

private String getToken() {
    // @formatter:off
    return RestAssured
        .given()
            .header("Content-Type", "application/json")
            .body(new JSONObject()
                .put("grant_type", "client_credentials")
                .put("client_id", 1)
                .put("client_secret", "Gsv7brPlt5fIpqa1ebFc5A1uHwDJBPY6TRaMm8wa")
                .toString())
        .when().post("https://raudhahku.com.my/api/v1/oauth/token")
        .then()
            .statusCode(is(200))
            .body("$", hasKey("data"))
            .body("data", hasKey("access_token"))
            .extract().body().path("data.access_token");
    // @formatter:on
}

Just write a simple reusable method to extract values using JSONPath and call the method in your code, here's a sample

Reusable Method :

public static String getJsonPath(Response response, String key) {
    String complete = response.asString();
    JsonPath js = new JsonPath(complete);
    return js.get(key).toString();
}

Test :

public static void main(String[] args) {

    Response res = given().header("X-API-KEY", Config.API_KEY).header("session", this.sessionID)
            .header("username", this.userNameId).queryParam("code", verifiedCode).log().all()
            .get(baseUri + basePath + "/vm1/verifyCode").then().log().all().extract().response();
    String value = getJsonPath(res, "accessToken");
    
    System.out.println(value);
}

Update :

public static String getToken(String key) {
    String res = given().header("X-API-KEY", Config.API_KEY).header("session", this.SessionId)
            .header("username", this.UserName).queryParam("code", verifiedCode).log().all()
            .get(baseUri + basePath + "/vm1 /verifyCode").then().log().all().extract().asString();
    JsonPath js = new JsonPath(res);
    return js.get(key).toString();
}

@Test
public void testAuthValidator() throws InterruptedException, IOException, GeneralSecurityException {
    String sentCode = GmailUtility.getVerificationCode();
    System.out.println(sentCode);
    String Token = getToken("accessToken");
    System.out.println(Token);
    driver = initializeDriver();
    driver.get(env.API_Website);
    AuthApi auth = new AuthApi(driver);
    auth.getAuthorizeButton().click();
    auth.getValueField().sendKeys("Token " + Token);
}

You can get any value using this