There is no beam or anything extra required; a conversation option should just show up if you have rope in your inventory.
Try selling all the rope you have and buying one back (or pick up a different piece of rope; there are a couple in the bandit hideout, if you didn't get those yet). Maybe that will fix whatever the glitch is with the game not recognizing that you have rope.
If that doesn't help, try emailing support@larian.com, with a description of the problem and maybe a link to this topic.
Welcome to the forum.
