PIL cannot write mode F to jpeg

Try convert the image to RGB:

...
new_p = Image.fromarray(fft_p)
if new_p.mode != 'RGB':
    new_p = new_p.convert('RGB')
...

Semente's answer is right for color images For grayscale images you can use below:-

new_p = Image.fromarray(fft_p)
new_p = new_p.convert("L")

If you use new_p = new_p.convert('RGB') for a grayscale image then the image will still have 24 bit depth instead of 8 bit and would occupy thrice the size on hard disk and it wont be a true grayscale image.

I think it may be that your fft_p array is in float type and the image should have every pixel in the format 0-255 (which is uint8), so maybe you can try doing this before creating the image from array:

fft_p = fft_p.astype(np.uint8)
new_p = Image.fromarray(fft_p)

But be aware that every element in the fft_p array should be in the 0-255 range, so maybe you would need to do some processing to that before to get the desired results, for example if you every element is a float between 0 and 1 you can multiply them by 255.