Installer App Missing from App Store
"macOS installer app" is Sierra, all prior OSes were "OS X". 

A unibody can't run Sierra [macOS].  

You need El Capitan [OS X], which can be downloaded from the App Store [though it's difficult to actually search for], especially kept live by Apple because of the 'break' in OS support that happened at Sierra. 

See Apple KB: Upgrade to OS X El Capitan which gives links that will open the correct location in the App Store app.

...or try this direct link