Cheetahs and snow leopards are two big cats that cannot roar, though they make other big cat noises such as chuffing and growling. The sounds a big cat can make depends on certain bone anatomy in the animal's throat, and neither the cheetah nor the snow leopard have the proper anatomical features to allow roaring to occur.
The exact reason for cheetahs and snow leopards inability to roar has to do with throat physiology related to the hyoid bone, which is a part of the throat anatomy and helps to determine what sounds a big cat can make. Both snow leopards and cheetahs can purr. Other big cats may also be able to purr, but may have different mechanisms for doing so.