Why can't I download an OSX update again (Sierra)

I'm trying to update an iMac at my work from Yosemite to Sierra (Sierra is the most recent version that is compatible with some hardware we use.) I followed a link to the App Store from Apples website to find Sierra. I clicked download and it started downloading.

When I went to install it, it told me the file was corrupt. I followed some advice to fix the install file that I found online that involved viewing the contents of the installer, and deleting some "permissions" files. This at least got the installer going, but it froze shortly after starting, before the progress bar advanced at all.

The next advice I followed was to delete the installer and re-download it. I deleted it, but now when I go to the App Store there is now "download" button, only an "open" button, which does nothing, since I have deleted the install button. I tried logging out of the App Store, no luck. I've tried rebooting, no luck. I've tried clearing the App store's cache, but there's nothing for the App Store in Library/Caches/

any help would be appreciated. It's a 2013 i5 iMac, and I have checked, it is compatible with Sierra.