Purify the Grave can’t be used in response to a spell being cast with flashback or a spell that requires cards to exiled from a graveyard as an additional cost to try and counter the spell or prevent it from being cast.

If it’s your turn, and you cast a spell with flashback from your hand, you’ll have priority to cast that spell again using flashback before any other player has priority to cast Purify the Grave in order to remove that card from your graveyard.