Here's why:
* Flash is a move that is not in Hoothoot's learnset. This means it can't learn it through leveling up, breeding, or via a TM/HM.
* Hoothoot is primarily a Normal/Flying type. Flash is a Normal-type move, but its type doesn't affect whether a Pokemon can learn it.
If you need to use Flash, you'll have to find a Pokemon that can learn it. Some examples include:
* Pikachu (learns it by level up)
* Goldeen (learns it by level up)
* Voltorb (learns it by level up)
* Squirtle (learns it by TM)
Remember, you can check a Pokemon's learnset online or in-game by looking at the move list for that Pokemon.