I have Moved the Sierra Finder into the coresystems folder and deleted the original mojave finder…
bryson lim

You need to relaunch Finder, you can do this by clicking Finder icon in Dock by right click while holding Alt/Option key. Please note, that you shouldn’t have deleted old Finder.app. This method is silly, you won’t be able to duplicate files and move them to trash. So keep the original Finder.app and swap them only when you have to work with Cover Flow.