why can't i download mac0s sierra

What have you tried?

If you do have an operating system on your mac and it is functioning then you can download

Sierra using this method.

How to get old versions of macOS – Apple Support

Go to Download macOS and click on macOS Sierra 10.12

This downloads InstallOS.dmg to your Downloads folder.

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

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

When downloaded open to get InstallOS.pkg, double-click on

that and an installation window will open, read and follow the prompts,

this does not install Sierra but uses the InstallOS.pkg to create the 

Install macOS Sierra.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 Sierra on, that is for later.)

To start the installation of Sierra double click on the Install macOS Sierra.app.

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

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

an external drive for safe keeping.