Select JPG from the Format drop-down menu in the Save As dialogue box.
You did something to the file that makes it unable to be saved as a jpeg.
JPEG only supports 8 bits/channel, in RGB, CMYK or Grayscale or a maximum of 65000 pixels either side. Those are the limitations of the file format.
If your file is Indexed color mode, it couldn't be saved as a jpeg. Also if the file has floating layers, you would have to flatten it.
If your document is not in those modes or depth, then it cannot be saved as JPEG, and thus JPEG would not be listed in the available file formats.
Correction: Photoshop will save a jpeg copy if the file the jpeg option exists.