you can’t use relative location for actor that is not a part of actor you want it to be relative to. in usual case actor is child of the world, so world location is like relative to world. do you understand? you need to add actor as a component to another actor, to make it relative. in other case you can set only world location, because actor is relative to world.
but maybe you can try to attach it to another actor, and use relative offset to simulate that sliding.