Create a High Sierra Bootable USB

I recently took possession of a 2010 iMac with a bad hard drive. I replaced the hard drive with an SSD, and am now trying to create a bootable USB to install High Sierra OS on the iMac. Using my Mac air, and following the instructions on the Apple Store page, I tried to download high Sierra, but each time my system preferences software update launches and I get a message that "OS cannot be found".

What am I doing wrong?