To install Nik software, Photoshop Elements 2020 is not recognized as a supported application on your Mac machine. That is, when your machine has Mac App Store 2019 and 2020 installed, the list of compatible hosts in Mac App Store displays only Photoshop Elements 2019.
To resolve the issue, add Nik plugin in the app preferences:
Choose Photoshop Elements Editor > Preferences > Plugins. Click Open Plug-Ins folder in Finder and add the Nik plugin. Click OK and relaunch Photoshop Elements 2020.