Download macOS High Sierra

If you have a brand new MacBook Air 2020 then it came preinstalled with macOS Catalina,

you cannot install an older OS than the one it came preinstalled with.

I understand you want to download High Sierra in order to create a bootable USB stick for use on another mac

but because of your mac being 2020 and unable to install High Sierra the App Store is

not allowing you to download because it knows what mac you are using and makes the assumption that you

want to install High Sierra.

When you go to the App Store High Sierra page what does it say.

Having a cabled connection is always more reliable, but you can try with WiFi, as long as you do not use the mac while it is downloading, or any other devices that share the same internet connection.

Do you have another mac that could run High Sierra or one that you can borrow.

You can buy a Thunderbolt to Gigabyte Ethernet adaptor from Amazon, for future use.

You can still download the full High Sierra install app from the App Store,

Use Safari as your browser and read this link.

How to upgrade to macOS High Sierra - Apple Support

Go to Section 4 and click on Get macOS High Sierra.

Connect your mac to your router using a cabled connection rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

The link does work.

What mac are you downloading this on and what operating system is it running.

Have you made sure your mac is connected to your router via cable and not WiFi.

As per the instructions in bold in my first reply.

When you get to the High Sierra App Store page it should say 'Get'.

If it says 'Open' then the App Store has detected a copy or partial copy of Install macOS High Sierra in your Applications folder.

If it is there move it to the Trash, empty the Trash. Restart your mac, and try again.

If you get any more error messages then post a screenshot here.