Earlier today, my Windows 10 Pro x64 PC updated to the Fall Creators update (version 1709). After doing this, I tried to install Classic Shell v4.3.1 and got the following error
"The system administrator has set policies to prevent this installation."
In my case, this problem was caused by a security program I use named CryptoPrevent. To be able to install v4.3.1, I temporarily set the Protection Plan in CryptoPrevent to "None" and then changed it back to "Default".
You may also want to look at an earlier thread with some more info - http://www.classicshell.net/forum/viewtopic.php?f=7&t=7811
Hopefully, this posting will help anyone else if they encounter this problem.