You sold the Chocobo Lure Materia and cannot buy another (Gold Saucer Battle Square only prize, 2000 GP). Without it, you cannot encounter Ruby Weapon on the world map (requires Lure + Gold Chocobo).
Bottom Line: Reload save or use Save Editor. The Lure is a Key Item — non-duplicable, non-rebuyable. The "glitch" to fight without it is not practical.