Are you talking about the old Nik Collection that was taken over by Google and distributed for free? I just installed it into Elements 2020 without a problem.
During the installation process there was a dialog with the found host applications on my machine. Elements 2020 is actually Elements 18. (Did you mistake 16 for 18?) If it is not finding the Elements 18 host on your computer, you can add it by clicking the + button and navigating to the Elements 2020 Editor exe file.