I receive the message "Taskbar clock replacement is not supported in the version of Windows" every time when run World Clock on Win10.
Could you advise how it may be fixed?
Microsoft removed the means to replace the taskbar clock properly in Win 10 since before their Anniversary Update. The warning shows up because you have at least one clock configured to run "in taskbar". To remove the warning, open Preferences and make sure that all clocks are configured to run "on desktop" as opposed to "in taskbar" - the operating system no longer supports "in taskbar" operation of World Clock, unfortunately.