The D3400 was first supported in Camera Raw 9.7 which is only supported in Cloud (CC) versions of Photoshop. The last version of Camera Raw compatible with Photoshop CS6 was 9.1.1 so you'll never be able to open D3400 Raw files in CS6.
You'll either have to
- join the Adobe Cloud to open Raw files in CC, or
- Download the free Adobe DNG converter, batch convert Raw files to DNG format, then edit the DNG files (not the Raw files) in CS6. An extra step for you but no extra cost.
Thank you. I did download the converter, but it doesn't see my raw images to convert.
Follow the video.
The DNG converter sees folders not files. You convert folders of Raw files to DNG files.