How are you downloading the OS, I hope not from Apple Recovery servers.
You can download High Sierra from here,
open Safari, do not use Firefox,
How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click no Get 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'.
Once the download has completed you will have Install macOS High Sierra.app in your Applications folder.
To make a bootable USB installer follow the instructions in this article.
How to create a bootable installer for macOS – Apple Support
As part of the installation process an installer window will ask you to select a destination disk
for the installation, this is where you can select your SSD drive.