Great question! The action can not be played due to the following:
The first sentence on Rope and Ride is: "React: After a dude accepts your callout, but before posses are formed, boot your leader or their attached Horse to move your dude then the mark to an adjacent location that is not your home."
Full Moon Brotherhood: Noon, Boot: Choose an opposing non-token dude that is adjacent to one of your dudes. That dude loses all traits, cannot move via card effects, and cannot refuse callouts. If that dude leaves play, either draw two cards or gain 2 ghost rock.
The first sentence is a requirement on Rope and Ride; as the requirement to move the dude cannot be performed, the action can not be played.
This can be compared to another React, with the card, Shan Fan Showdown. "React: After posses are formed, choose your dude with a Melee Weapon in your posse." If you don't have a Melee Weapon in your posse, the card cannot be played.