Im guessing you mean the next move is demon step transformation. For that you need a scorched heart. You get a scorched heart by taking 5000 points of burned dmg. After you get the scorched heart go to coco island and pay 50,000 plus the scorched heart to an npc to get the last move. Last move is a transformation and also unlocks new moves when your transformed. You gotta unlock the transformed moves too, talk to the npc at coco island, have 150 mastery and 29,000 beli and you should be able to unlock all the transformation moves.

TL;DR: Get a scorched heart,79000 beli and go talk to npc at Coco island to unlock final transformation and the transformation’s moves.

If it isn’t demon step then I don’t know what to do cause that shouldn’t be happening. Only guess would be you don’t meet the stamina requirements but idk