Not necessarily.
http://games.chruker.dk/enemy_territory/entities.php?action=show_entity&entity_name=script_mover
An origin brush is needed only if a model is supposed to be attached to the entity, for example if it’s supposed to be a vehicle. The script_mover would be the invisible clip, the “model2” key specifies the model to attach to the clip and the origin brush specifies where exactly. The middle of the cube will be set as the model’s origin, hence the name.
A script_mover without a model, for example a solid sliding door, doesn’t need an origin brush. So maybe, if your mover contains an origin brush, the engine requires you to also set the “model2” key or it will print your error message. If you don’t need a model, try deleting the origin brush.
Also, make sure the mover and the origin key are part of the same entity, not two separate entities with the same name. You can check this in your map editor, by selecting a brush of an entity and then choosing ‘Select entire entity’ or something like that. This should select both your mover AND your origin brush.