You need to synthezise all 18 or 21 items first if it is already there maybe you have some
missing ingrediants.If you synthezised all 18 or 21 items and it is still not there talk
to the moogle again and look at the bottom you will see the Ultima Weapon.

It's very hard to make one. first you must make All the things you can make. then get the
stuff for the weapon. then u can make it