How to install Windows 7 onto MacOS Sierra without downgrading using BootCamp? How do I do a partitioned install of Windows7 on my MacOS sierra? 

Sierra won't let me use boot camp assistant. I have a macbook air (late 2015) 8gb ram model. Any help would be appreciated.
Windows 7 is not supported by Apple with your hardware.
The latest MacBook Air compatible with Windows 7 64 Bits is the Early 2014 Macbook Air.

You can install Windows 8.1 or Windows 10.

Check your system compatibility on this Apple Support Page.