The old OS's are available.
The Combo update is just that an update for which you need a previous version of High Sierra
to already be installed on your mac.
Moving from one OS to a newer OS is called an upgrade.
What you need is the full installer which can get from here,
Please use Safari to connect to the link.
How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click on Get macOS High Sierra,
this will redirect you to the App Store High Sierra page
it should say Get or Download
this will download the full installer to your Applications folder
it should be 5.24GBs in size.
The version of High sierra is the last/ latest version available 10.13.6.
When Apple updates its OS's it removes the previous full version from the App Store,
When 10.13.6 was introduced, 10.13.5 was removed as was 10.13.4 before it and so on.
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'.