How do I download OS X to external hard drive?

I have erased the hard drive in a mid 2014 MacBook Pro that I am selling. When I tried to download a new OS X I got a circle with a line through it on a black screen. I have just erased and reformatted a 500 GB external drive.

When I go to support, all I see are updates for OS X. How do I get the whole OS and reinstall it on the Mac Book Pro? I'm using the lates OS X on a 2016 Mac Book Pro.

tia