Trying to re-install OS X Yosemite and it says “this item is temporarily unavailable.”

So I recently got a iMac from my friend and I factory reseted it so I can turn it into my computer cause all of his stuff is on there and I try to reinstall os x Yosemite and everything is going fine until it ask to sign into the App Store and I put in my info and it’s says “this item is temporarily unavailable.” Can anyone please help me

[Re-Titled by Moderator]