Here's why:
* The Escape Rope's functionality: The Escape Rope is designed to allow you to leave a wild encounter without battling. It's mainly useful in areas with annoying or difficult wild Pokémon.
* The Indigo Plateau's nature: The Indigo Plateau is the final challenge of the game. It's a series of trainer battles, not wild encounters. You are expected to engage in these battles to progress through the story.
Therefore, you cannot use an Escape Rope to avoid battles in the Indigo Plateau.
If you want to escape a battle in the Indigo Plateau, you'll have to defeat the trainer. Good luck!