The message Error 312: The specified Security Code was invalid appears because the security code from your Authorize.net account can not be sent to 123FormBuilder. To prevent this problem, you’ll need to disable the Security Code option in Authorize.Net. This will solve the problem with the invalid security code error message on the payment page.
Log into your Authorize.Net account and click on Account. In Settings, go to Transaction Format Settings → Payment Form and click on Form Fields.
Scroll down through the options and uncheck Require the Security Code feature on the Payment Form and hit Submit. This will prevent the error to occur in the future.