I have a Nikon D5500 and viewing a JPEG is fine. If I change my file format to NEF(RAW) when importing files lightroom cannot view them. It gives me a box to check for updates but I have to most recent version. Ver:5.7.1.
A few weeks ago I used my friends Nikon D5300 and the RAW files were perfectly fine.
It's a new camera model that was released in that "hole" between Lightroom releases. But you can download the DNG converter version 8.8 and convert that NEF files to DNG. I know, it's a Band-Aid fix. But it will work in the meantime while we wait for the next Lightroom update or upgrade.