A weapon must have an inscription slot to be able to put an inscription on it.
Weapons dropped in Tyria or Cantha never have inscriptions or inscription slots. Furthermore, not all weapons dropped in Elona have insciption slots.
A weapon has an inscription slot if the weapon description reads "Inscription: None", or if the weapon already has an inscription on it (which can be salvaged or over-written).
For further details, see Inscription