Can’t download High Sierra installer

The full installer is what you need. The smaller file version is not a full installer! It’s a limited maintenance updater (after you have the full version installed).

To get the full version you need to go to the AppleStore and look at your installed app’s listing (older App Store app) and then download if from there. If you hadn’t installed it in the past or have the newer Store app. You’ll need to download it from within the internet recovery or recovery partition. Or, if you have a friend who has the installer!

Follow this guide to create a workable OS installer USB thumb drive How to create a bootable macOS High Sierra installer drive

To be honest, unless you have a strong reason staying with High Sierra I strongly recommend going to Mojave instead! Its just better! How to create a bootable macOS Mojave installer drive