The most likely answer here is that your laptop does not feature an optical drive, while it has the opening for one, it and it's associated hardware was never installed.
I only found one out of the five Aspire ES1-512 models I checked that features an optical drive (model Aspire ES1-512-P9GT) so if that's not your model, you probably don't have one
I could find no information as to whether the hardware is there to install one on the other models (I'd suspect not though)