No, a massless string cannot exert a normal force on a wall. A normal force is a contact force that acts perpendicular to the surface of contact. Since a massless string has no mass, it cannot exert any force on the wall in the normal direction. It can only transmit tension along its length.
A massless string primarily exerts a tension force along its length. When attached to a wall, the string can pull on the wall with a force equal to the tension in the string, but this force acts along the direction of the string, not perpendicular to the wall.
Yes, the tension in a massless string can affect the wall it is attached to. The string will exert a pulling force on the wall equal to the tension in the string. However, this force is not a normal force; it is a tensile force acting along the direction of the string.
A massless string would exert a force on a wall if it is under tension. For example, if an object is hanging from the string, the tension created by the object's weight will pull on the wall. The force exerted is along the direction of the string and is equal to the tension in the string.
In theory, a massless string itself cannot cause deformation in the wall because it has no mass. However, the tension transmitted through the string due to external forces (like a hanging weight) could potentially cause deformation if the tension is large enough. The deformation would be due to the tensile force, not a normal force.