It means he couldnt kill himMard Geer - "I'm going to kill you"
Zeref - "I'm afraid that's not possible for you"
gee I wonder what that could mean
maybe that Mard Geer can't kill him with anything he does?
AND WE DONT KNOW WHY
Zeref could mean that if he used Memento Mori he could just get out of its range or something thereby rendering it useless