I can't buy assassin's gambit

I am struggling to kill Morgott The Omen King because I can't sneak up on him. I went to Volcano Manor and spoke to Recusant Bernahl but there's a 0 on Assassin's Gambit.

I am struggling to kill Morgott The Omen King because I can't sneak up on him. I went to Volcano Manor and spoke to Recusant Bernahl but there's a 0 on Assassin's Gambit.