From the message I saw when I tried downloading ASOT 100, it appears that the "service" is temporarily unavailable. In other words, the host where the Podcast is located is not working correctly, the episode cannot be found or some similar fault has occurred.
I suggest you try again tomorrow. If the problem continues, contact the Podcast's producer.
I've just tried ASOT100 again now, but the same problem occurs. I suggest you contact the Podcast's producer as they appear to have a problem which they may not yet know about.
After several more attempts, episode 100 is now downloading to my iTunes but fairly slowly. It's a special episode (and it's two hours long) so perhaps over-demand is causing the download to fail.
As I'm typing, the remaining time to download is actually increasing and that makes me more certain that it's simply more people trying to download it than the host server can supply.