Hi Bikrr -
I'm surprised to hear you are experiencing this behavior. All prototype URLs are public and there are no restrictions that should prevent you from access it.
My guess would be that you have the wrong URL (perhaps with a character missing or something like that). Can you double check to confirm you are using the same URL as the prototype's author?
If you can send the URL you have to AdobeXDSharingSupport@adobe.com, I can investigate further and tell you exactly what is going on.
Sorry for the late follow-up, but to answer my own question...after some troubleshooting, the XD author restarted his Mac and re-generated the link and everything worked fine.