A couple of Photoshop pointers,....
If you have a multi-layers file, it can't be saved as a .JPG. ----> Flatten the image first.
[ Layer --> Flatten Image ] or [ Shift/Command/ E] to merge visible layers.
If your file is in a 16 bit mode, it can't be saved as a .JPG. ----> convert to a 8 bit/channel mode first.
[ Image--> Mode--> 8bit/Channel ]
If you want to re-size after or before saving. --> [ Image --> Image Size --> pop-up box --> set size as desired --> Return ]
Remember to change the file name after re-sizing, to prevent overwriting a large file with a new smaller file of the same name.