I had the same issue. However, the 'sudo softwareupdate' solution did not work. I was having two admin accounts. One is local to laptop. And another is my domain user account (mobile/Admin). The issue was happening with my domain user account. I was able to install once I switched on to local admin account.

Had the same issue. I am happy to mention this in the past tense. After multiple troubleshooting, I got it working. The problem I had was that the spotlight will not list anything except 'search in web'. The finder items, applications, suggestions, everything was missing. Apart from that Apple Mail was unusable since the Smart folders and search were not working. I tried, "sudo mdutil -E /" "sudo mdutil -i off " and "sudo mdutil -i on /" and still it was not working. However, my account that I had this problem was a Mobile Admin account that is bound to my network. I never had this problem in previous upgrades for the whole decade. Solution that worked for me: Tried the same mdutil commands displayed above, but I did it as another local super user who is also an admin. I'm logged in with the mobile admin account. In the terminal I did "su <>". Then tried "sudo mdutil -E /" "sudo mdutil -i off " and "sudo mdutil -i on /" That's it... after the above commands life was back to normal.