You must batch convert a folder of NEF files into DNG. Launch the DNG Converter app. Select your source folder with the NEFs and choose a new destination folder for the DNGs. When conversion is complete import the DNGs from the new folder.
- which ACR converter version is required for you camera
- which ACR version your Elements version supports.
The general rule is that if the camera is newer than the PSE version, you can't update because each camera model comes with its own unique raw format.
If you don't want to pay the upgrade to the current PSE2019 version, you can download the free Adobe DNGconverter software mentionned in the above faq. That standalone program (not a plugin) can batch convert a whole folder of your NEF files to the DNG raw format that your PSE15 can read and edit. An additional (batch) step, with the same quality.