If somebody installed Classic Shell on that machine, and you don't want it, simply uninstall it. There is no harm done.
If you want to use it, then click OK - it will then ask for admin access, click Yes instead of No. However seeing that you have version 4.2.1, I would recommend instead to just install the latest version 4.2.5.
The reason you are getting this message is that Windows got upgraded to a new version (most likely the November update of Windows 10), which destroys some important settings of Classic Shell. Part of the setup needs to run again to repair it.
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