App store doesn't have High sierra available for download.

High Sierra is still available for download and you get it through the App Store.

Open your Safari browser and click on the link below, Firefox will not work.

How to get old versions of macOS – Apple Support

Check that your mac can install High Sierra and if it does scroll down to

Download OS and click on macOS High Sierra

this will redirect you to the App Store High Sierra download page it should

say Get or Download.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable 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.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

I'm in the US, specifically Pa.

Are you using a VPN or a security app? If so, try while in Safe Mode.

How to use safe mode on your Mac - Apple Support

That's weird. Works here…

It is available here in the UK are you using Safari to click on the link.