Aperture 3.6 has only been available from the AppStore, never as a download and update to the versions, that have been sold on installer disks.
You have waited too long. When Apple moved Aperture to the App Store when the Aperture 3.5 update has been released for MacOS 10.9 Mavericks in 2014, there has been a short window of opportunity, when we could transfer the older Aperture Licences to the App Store and get an AppStore licence for Aperture. This is no longer possible, because the development of Aperture has been stopped in 2015, and Aperture is no longer sold.
Now you can only get a copy of Aperture 3.6, if Apple Support is willing to help. If you just got yourself a new Mac and need to set it up, Apple Support may be willing to help and provide an Aperture 3.6 installer, as you own an Aperture 3 serial number.
The Aperture 3.4.5 updater is the last update that has been offered freely for downloading from Apple's download page. It is just an Updater, not a full installer. You need first to install Aperture 3 from your installer disks, if you can get the installer to run. The certificate may have expired. And the installer may contain legacy code, that cannot run on El Capitan.
But Aperture 3.4.5 will not help on your current system, and even of you downgrade to MacOS 10.8 Mountain Lion, so you can run Aperture 3.4.5, it will crash constantly, unless you are keeping the Mac offline, because the code needs access to the old Google Maps that are no longer available.
You can open your old Aperture Libraries in Photos for Mac to get access to your photos, while you decide, which application you will use as a replacement for Aperture in the long run. See: Migrate your Aperture library to the Photos app or Adobe Lightroom Classic - Apple Support
