cannot write mode RGBA as JPEG

Question

JPG does not support transparency - RGBA means Red, Green, Blue, Alpha - Alpha is transparency.

You need to discard the Alpha Channel or save as something that supports transparency - like PNG.

The Image class has a method convert which can be used to convert RGBA to RGB - after that you will be able to save as JPG.

Have a look here: the image class doku

im = Image.open("audacious.png")
rgb_im = im.convert('RGB')
rgb_im.save('audacious.jpg')

Adapted from dm2013's answer to Convert png to jpeg using Pillow