older .nef files

In case the nefs have been corrupted by Nikon transfer:

https://exiftool.org/fix_corrupted_nef.html

I don't use Nikon but I remember seeing old discussions about NEF files which could not be opened even if they opened previously. It was found to be NEF files which had been processed in the meantime with a faulty Nikon transfer software. A solution was quickly found, so it may be difficult to find the relevant discussions.

It depends on the camera model that the NEF came from. You can check supported camera list here:

https://helpx.adobe.com/camera-raw/kb/camera-raw-plug-supported-cameras.html

Please review this:

FAQ: Will Photoshop Elements work with my camera, ... - Adobe Support Community - 4303022

You may need the DNG plug-in if your camera is not compatible with PSE,