This is because JPEG is not a native file format in Photoshop. Photoshop can open and edit JPEGs, but it cannot save images as JPEGs. To save an image as a JPEG, you will need to use a third-party software application or convert the image to another file format such as PNG or TIFF.
You'll need to resize the image or crop it down to reduce the file size. The image has too many layers: Photoshop can only save JPEGs with a maximum of 10 layers. If your image has more than 10 layers, you'll need to delete some of them or flatten the image before you can save it as a JPEG.