The EM field itself is quantized. You start with the classical electromagnetic field and you try to quantize it. The rules for this are not precise nor formal

The EM field itself is quantized. You start with the classical electromagnetic field and you try to quantize it. The rules for this are not precise nor formal. That means something a little different to "photons can only have specific energies" (that applies only to very specific atoms or atoms with certain structure; in many materials, electrons can exist in bands of continuous energy).