PS v25.6 Can't save NEF as JPEG

Check the bit depth in camera raw output preferences. 

Under the Save menu, Photoshop now offers only file formats that support all elements of the file. For jpeg that means 8 bits/channel, no layers, RGB, no additional channels, no transparency.

If your image does not fit that format, for example it is 16 bits/channel, then jpeg can be found under Save a Copy.

Dave

I have a similar thing: After creating a new file it is not possible to find JPEG-file format in the save as dialogue. Only seven format are listed: JEPG 2000, PSD, IFF, PDF, PNG, TIFF, and WebP. I have had a discussion within a facebook group. There is a user encoutering the save error as you do. When I open a RAW (NEF) I'm able to save it as JPEG. 

I went to edit > preferences -> file handling  and acitived legacy saving. This worked fine for my problem, but did not solve the problem of the user having the issue as you do.