Reinstalling OS X Yosemite

There is no need to go to the App Store to get Yosemite,

click on this link,

How to upgrade to OS X Yosemite – Apple Support

Go to Section 4 and click on Get OS X Yosemite.

This downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running Yosemite,

a mac that came preinstalled with an OS later than Yosemite will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install Yosemite

but converts the InstallMacOSX.pkg to the Install OS X Yosemite.app which 

you will find in your Applications folder.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Yosemite on, that is for later.)

To start the installation of Yosemite double click on the Install OS X Yosemite.app.

The copy of the install app self deletes after installing Yosemite, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X Yosemite.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk using the Install OS X Yosemite.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

I've seen this before. It turned out (at the time) that it was only unavailable to that Apple ID. Asking someone else to log in with their Apple ID and password fixed it. Of course, you get them to type their details so they remain confidential!