If you've purchased iPhoto or Aperture from the Mac App Store, you might notice it's missing from your Purchased list.
*Update: The missing previously purchased but no longer for sale apps have been restored, and you should once again be able to find them in your purchased tab in the Mac App Store. *
Both apps, along with many other no longer for sale apps, recently went missing from the Purchased list of Mac App Store account users. But don't worry, they'll be back. Sources indicate that Apple knows about the problem and is taking steps to correct it.
In the interim, if you have iPhoto or Aperture currently installed on your Mac, you need to do precisely nothing. Both apps work just fine and will continue to work in the future.
If you're trying to set up a new Mac install with these apps, however, you've found that you can't. And you won't be able to until Apple resolves the Mac App Store backend issue that caused the glitch. The glitch has apparently also deleted versions of OS X prior to Yosemite. Mavericks, Lion and Mountain Lion were all distributed via the Mac App Store.
iPhoto and Aperture were both discontinued earlier this year when Apple released Photos for OS X Yosemite. While Apple no longer sells the apps, it continued to make them available for download for users who had bought them. At some recent point, those links — in Mac App Store users' Purchased list — disappeared.