This typically happens when the event is still in Under Construction mode. You can take the following steps to move the Kintera Thon event to Preview mode, which will allow you to view the URL:
  1. Go to Communities > Special Events > Friends Asking Friends > Kintera Thon
  2. Hover over your Kintera Thon event and click Website Info
  3. Click the Preview radio button
  4. Click Publish This Event Now
If the even is in preview mode or active mode and is still experiencing this message, attempt to view the page outside of your organization's network.

If the error is occurring across multiple networks, Chat with Support and reference this article.