How can I download InDesign onto my OS X Yosemite MacBook Air?

you could still install a 2017 version.

The current versions of Creative Cloud require 10.11 at a minimum.

To install InDesign, your computer must meet the minimum requirements for Adobe InDesign CC 2018, shown below. Note that Yosemite is not supported, so that's already an issue. I don't know the other specs for your computer, but you need to ensure that they meet the requirements shown in the first five bullets. GPU performance requirements are marked as optional.

• Multicore Intel processor
• mac OS version 10.13 (High Sierra), macOS version 10.12 (Sierra), or Mac OS X version 10.11 (El Capitan)
• 2 GB of RAM (8 GB recommended)
• 2.5 GB of available hard-disk space for installation; additional free space required during installation (cannot install on a volume that uses a case-sensitive file system or on removable flash storage devices)
• 1024 x 768 display (1280 x 800 recommended) with 32-bit video card; supports Retina display
• Optional: To use GPU Performance, your Mac should have a minimum of 1024 MB of VRAM (2 GB recommended), and your computer must support OpenGL version 4.0 or greater.
  • Supported machines
  •     iMac 4K
  •     iMac 5K
  •     MacBook Pro Retina
  •     Mac Pro connected to a HiDPI monitor
  •     Mac mini connected to a HiDPI monitor
  • To check for VRAM value, choose Mac > About This Mac (Graphics information).
  • To find out if your computer supports the required OpenGL version (4.0 or later), see this Apple Support document.