Changing Weapon Elements of Non-Element weapons???

So i bought serpent blazes from the unique badge shop only to find out it has no element. Proceeded to buy an element change (first time trying to change elements) only to find out cant use said element change. So can you only element change if at least on of the weapons has an element? Also bought a second serpent blazes just in case i needed the same weapon but ofc it has no element.

Also on a side note, do the serpent blaze even work for etoile? the normal atk deals less damage than my atlas ex

to change elements of weapons you need to do the enhancement of then, there you gonna find the option like is in the image