What do you mean by API Security?

API SECURITY

It is an additional security. An option that allows you to send your messages only through whitelisted IPs. It will also show IPs from where the APIs are being called.

If API security is enabled and you try to send the SMS via any other IP then those requests will be rejected with  error code 418.

If you do not wish to have this security feature, you can disable it by following these rules:

And now you can disable the API security option here on this page:

Enable your API security and then add your IP to whitelist it.