You cannot import Nikon D810 nef files into Premiere Elements 12. For that you need to have Camera Raw at least 8.6 which is supported by Premiere Elements 13. So, if you want to import Nikon D810 nef files into Premiere Elements, you need Premiere Elements 13.
However, if you convert your Nikon D810 nef files into dng files with the DNG 8.6 converter, you can import those dng files into your Premiere Elements 12.
I road tested the above to confirm. It worked for me. Please let us know if the above worked for you.
It worked. Thank you.
Thanks for the follow up that the suggestions worked for you. Great job.