High sierra download not working on sierra

I am attempting to download the a program on my old work computer that is running Sierra 10.12.6 and when I attempted to download High Sierra, which I believe I ended up being able to download an update to High Sierra and not the actual operating system, I am consistently getting a message saying I must download 10.13 first. Every time I have attempted to download High Sierra, I'm getting an error message saying "the copy of the install macOS High Sierra application is damaged, and can't be used to install macOS." Any idea how to work around this?