Walls are unseen barriers that Reapers] can set up in the way of your path. If you are blocked by a wall, try and find the Reaper nearby. They will always be wearing a red or black jacket. Talk to them, carry out their demands, and talk to them again. If you've completed the mission, the Reaper will clear the wall. These Reapers are known as Support Reapers. The missions usually have the same ideas with them. The most common missions include: Making a certain brand popular, defeating a certain amount of noise in a chain battle, wearing a certain type of clothes, fetch quests, and the Reaper Review.
If a wall is up and there is no Reaper nearby, the area you are trying to access is not available for the day or mission. Find another route.