How to create bootable flash drive from macOS High Sierra combo update I have problem installing macOS High Sierra from App Store. I don't understand what the problem is, but when the download completes it suddenly resets. Then when I open up App Store again it only lets download the os again.

I want to clean reinstall High Sierra on my Mac and the only way to do it is to create bootable flash drive because I have a bad internet connection. 

I have downloaded macOS High Sierra combo update with a download manager, and my question is can I create bootable flash drive with this combo update?
No, you cannot create a macOS USB Installer from the Combo Update, you need the full Install macOS High Sierra.app to do that.

Have a look at: How to create a bootable installer for macOS

If you cannot get it to download properly on your system using your connection, then you'll have to try from a different location and connection or have a friend download it, or go to an Apple Store.