Want to call Installment update REST API from PL/SQL

We have a requirement to update Payment method of an AP Invoice Installment via REST API call from PL/SQL(PaaS DB). We are able to execute a normal url, but the API cal does not returning anything. Not sure if I am missing anything

Steps we followed:

1. Created ACL
2. Created Wallet
3. added certificate to the wallet. (Certificate we created from the instance url)

Below is the code snippet:

create or replace package body xx_cerp.adg_rest_api_test

IS

  PROCEDURE update_installment (

    p_invoice_id   IN NUMBER,

    p_installment_num IN VARCHAR2

  ) IS

    req   utl_http.req;

    res   utl_http.resp;