Dispatch states it requires a one-handed weapon to use, however when Fangs of the Father are equipped (Main Hand/Off Hand) it states requires One-Handed Melee Weapon.
According to Blizzard’s reasoning, you can’t use a dagger mainhand for Dispatch. Dispatch requires a one-handed sword, one-handed axe, one-handed mace, or fist weapon in mainhand.
Daggers do not count, and it’s deliberate so not a bug.