It's a permission issue. Can you please give 775 permision inside your custom folder of suitecrm root directory. If still facing an issue then email to us on suiteplugins@gmail.com
Once you configure step 3 in link https://store.suitecrm.com/docs/webtolead/admin-guide you will get WebHook URL in form. You need to post your form data to this URL from your form of website as well as your wordpress form.