[n-a] Senju Tobirama

Flying Thunder God Mark replaces to Flying Thunder God Mark Slash and that skill is only usable if you have a stack of Flying Thunder God Mark already active on an enemy. So it won't do anything if you don't already have a stack on the enemy since it can't be used. 

So you want to have a stack on the enemy and then use it on yourself. Otherwise it's not gonna be usable even if you use it on yourself.