"Item not available" error when trying to upgrade to High Sierra
I would recommend opening these links in Safari so they translate easier to the correct App Store app.

https://apps.apple.com/app/macos-catalina/id1466841314
https://apps.apple.com/app/macos-high-sierra/id1246284741

I removed the US store from the High Sierra link and cribbed the Catalina link from another answer. Also, once you get the download set, other issues can crop up. This following link is a compendium of knowledge and is super useful for reference.

How can I download an older version of OS X/macOS?