Have you by chance entered hardmode before installing Catalyst into your world? Catalyst makes it so that the astral infection spreads deeper on spawn, which is the area where Metanova Ore spawns. Since you've defeated Astrageldon, though, you should be able to find Metanova Slimes in the ug astral and get the ore that way

Nope, I installed all the mods i planned to use from the beginning and then proceeded to create the world im on. But yeah ill just use zerg potions in order to get some metanova slimes to spawn.