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iPhoto and Aperture were both discontinued earlier last 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.
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.
However, you can try downloading and installing it from the link below.
If you launch the new Photos program it will migrate your iPhoto library so you can use it with Photos.
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