There are a few reasons why Photoshop may not let you save a file as a JPG. One reason is that the file may be too large. Photoshop has a maximum file size for JPGs, and if the file is too large, it will not let you save it as a JPG. Another reason is that the file may not be in the right format.
Photoshop can only save JPGs if they are in the RGB color format. If the file is in another color format, such as CMYK, it will not let you save it as a JPG. Finally, Photoshop may not let you save a JPG if the image contains transparent areas. JPGs do not support transparency, so Photoshop will not let you save an image with transparency as a JPG.