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.
How to save a Photoshop File as a JPEG?
Try restarting Photoshop and see if that solves the problem. If not, you may need to uninstall and reinstall the software. The most common reason why a Photoshop file won't save is because the document size is too large. If your document exceeds 300,000 pixels in either dimension, you will not be able to save it.
Why won't Photoshop let me save a File?
You are not able to save your file as a PNG in Photoshop because your document is in CMYK color mode or is set to 32-bit channel. You can confirm this by going to Image>Mode… PNG files can only be saved in RGB and only support 8-bit and 16-bit channels. To save it as a png, go to File>Export>Export As…