Cannot reinstall OS 10 on MacBook air

I have a n older MacB Air that I have erased and am trying to reinstall OS X from the internet. From Disk Utility when I try to Reinstall OS X it opens Window to install Yosemite , after clicking continue I am told Apple will verify my computer’s eligibility...and then I get an error msg that a required download is missing. I have verified had and am told everything ok...how do I find what download I need and how to get it?