Plum IVR Support

We are attempting to use the validator for a remote Url (https://invoices-sandbox.epaypolicy.com/ivr/start) and we are unable to get it to validate. We are getting the error below -

This document is not valid VoiceXML!
Empty string supplied as input

Can you please help me understand why this error is occurring and what we can do to correct it?

Hello,

Our VXML validator does not currently support TLS 1.2 at this time. We have verified that your application code does properly validate using HTTP. Our IVR systems all support TLS 1.2 so you will be able to use that URL as your start URL when attempting to place calls.

Regards,
Plum Support