Here's why:
* Avifors are not a spell in the game: Avifors is a spell from the "Harry Potter and the Order of the Phoenix" game, not "Goblet of Fire."
* No modding support: The PC version of "Harry Potter and the Goblet of Fire" does not officially support modding, so you can't add new spells like Avifors.
However, if you're playing the game on a console, you can find a cheat code that will enable you to use any spell in the game, including Avifors (although this is not an official spell).
Please note: Using cheats can disrupt the game's intended balance and story, so use them with caution.