Lightroom Classic Plugin Problems

Did you specifically set Lightroom in the install?  (See this old forum thread)

https://community.adobe.com/t5/lightroom-classic/nik-collection-and-lightroom-classic-cc/m-p/9479374#M80189

Also-

NIK Plugins (except for Nik HDR Efex) must be added to the 'External Editors' in the Preferences dialog. If they install correctly they are accessible by the [Edit-In] command, and do not appear in Plugin Manager

Also they will install in the default Presets folders, so if you [Store Presets with the Catalog] you will need to copy the presets to the Catalog folders.

Check in [External Additional Editors] that you also have a compatible file-type set for each Nik program (JPG or TIFF).

Now, an inquiry, are you running MACOS Catalina?

If so, possible application rights issues, easily fixed.

Typically, on the MAC, in the MACOS, not in Lightroom, bring up System Preferences, select Security and Privacy, select the Privacy tab, scroll down to Files and Folders, make sure Lightroom Classic, and Photoshop have all boxes checked.

Also, if you place photos in the MACOS Photos folder, you may need to do similar for Photos

https://nektony.com/duplicate-finder-free/folders-permission 

Next up NIK, basically just reinstall NIK right on top of itself, the install script will invoke plugin install. You might want to double check you have the latest NIK update (if DxO) while you are at it.

Theoretically you could do this manually, but far easier to just re-run the install.

Followup/My bad, I see you did try to reinstall the plug-ins amd apps. Should always read twice.

First up Lumnar. Fortuantly Luminar includes a menu option to install plug-ins. You might be prompted for Admin rights.

In Luminar, up in the menu, click on File, click on Install Plugins, Install those you need. (this from a Windows PC, MAC may be slightly different)