What do you mean by API Security?
It is 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 removing the checkmark at the "ENABLE ADDITIONAL SECURITY" option.
Now if you need to enable the API security option please, checkmark the "ENABLE ADDITIONAL SECURITY" option.
And press Yes on the popup window:
Enable your API security and then add your IP to whitelist it: