installassistant_plain not downloading

I have a mid-2013 MacBook Air currently running on Yosemite (10.10.5) and am trying to upgrade to High Sierra. When I go to the app store, I start the update and it asks for my admin password for "installassistant_plain" to add a new "helper tool", which i've read has something to do with Siri. I enter my password and the progress bar gets less than a quarter of the way and then doesnt move. Nothing i do allows this download to progress and it means that i cannot get to the actual OS download. There are 30GB free on my hard drive so i know that this isnt the issue. Anyone else had this problem??