Grim Reaper's Sprint

Morbid — This spell costs {3} less to cast if a creature died this turn.
Enchant creature
When this Aura enters, untap each creature you control. If it's your main phase, there is an additional combat phase after this phase.
Enchanted creature gets +2/+2 and has haste.

Fallout

Morbid — This spell costs {3} less to cast if a creature died this turn.
Enchant creature
When Grim Reaper's Sprint enters the battlefield, untap each creature you control. If it's your main phase, there is an additional combat phase after this phase.
Enchanted creature gets +2/+2 and has haste.

If the creature Grim Reaper's Sprint would enchant is an illegal target by the time Grim Reaper's Sprint would resolve, the entire spell doesn't resolve. It's put into the graveyard from the stack, so its triggered ability won't trigger, and you won't untap your creatures or get an additional combat phase.