Hi, I had the same issue, but I think I figured it out. My internet was the issue. I have high-speed stable internet, but for some reason, it wouldn't allow me to install the update. When I went to my friend's house and tried his internet to download the beta it worked! My internet doesn't work with certain websites (I have no clue why) so it probably couldn't connect/work with Apple Software servers. I highly recommend you try a different internet connection. Try a friend's house or public wifi.