I was playing a Max Raid battle with other people, and during the battle I went for a Knock Off in Psychic Terrain, but it didn’t work. Why? Note: The Pokémon’s barrier was up at the time.
When used in a Max Raid Battle, Knock Off fails completely, without even dealing damage.
This is presumably to stop you from interfering with items in the Raids, as all moves that interact with items (Bug Bite, Fling, etc.) also fail in Max Raids.
Source
Hope I helped!
