According to this page, you need Camera Raw version 9.7 to support NEF files from a Nikon D3400.
Unfortunately, PS Elements 6 is so old that I very seriously doubt whether it will allow you to update to ACR 9.7… đ I think your best option is either to download Adobe DNG converter and convert your NEF files to DNG, then use ACR to edit them, or to use the RAW conversion software that came with your camera. I don’t remember what Nikon’s software is called, something like “Capture NX”. (See here for download details if you don’t have the CD that came with the camera…)