When I clean installed Win 10 creators update I found a setting which I hadn't noticed before. I too found that I couldn't install any of my programs & went looking for a setting, I found one concerning only allowing downloading/installing of apps from the MS App Store, I changed it to allow all installs & I could then install my programs.
Setting is found under 'Apps & Features'.
There is that, too, but I'd find it really unlikely that any company issued machine is on Creators Update yet.
It appears that the "Installing Apps" setting on the Apps & Features Pane of the Apps Settings gets initially set up based upon the UAC level you have set. If it's at the default or the highest setting (one notch above default) the "Allow apps only from the Store" level gets set. If the UAC is either off or at the level where the screen is not dimmed the "Allow apps from anywhere" level gets set.
This was another of those really annoying new settings about which no information was offered as part of the installation of Creators Update or immediately upon completion of the installation. I also had Fast Startup turned on again, and I am certain I turned it off, but this appears to have occurred with a very late update to the Anniversary Update (Version 1607) version of Windows 10.
Microsoft needs to stop resetting settings that the user has chosen to set to something other than their preferred defaults. They also need to start giving users lots of information about new settings (the Apps and Gaming Settings are entirely new with Creators Update) when these are introduced.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story