The condition for the Battle Task ""Knock Out your opponent's Active Pokémon 1 time with an attack from a Fighting-type Pokémon"" is to knock out a Pokémon in the Active Spot.
Moreover, the Pokémon in the Active Spot is considered as Active Pokémon, and if the Pokémon leaves the Active Spot, the relevant Pokémon will not be considered as Active Pokémon.
Therefore, if you knock out the opponent's Pokémon with the Grapploct's attack ""Knock Back,"" the opponent's Pokémon will receive the damage in the Active Spot while being knocked out in the Bench; thus it will not be counted towards the relevant Battle Task.