Why am I unable to equip some of my decals in Rocket League?

Each car body in Rocket League uses one of the six hitboxes and there are decals that are restricted to a specific hitbox and/or car body.

Most decals will have the name of the hitbox/car body upon which they can be equipped in front of their name.
Examples: “Octane: Skulls”, “Merc: Skulls”, "Aftershock: Tiger Tiger"

For more information, see Rocket League Car Hitboxes