I have referenced StackExchange post as well as a post from the Blender Forum. However, unlike their case, my output node seems fine and I only have one UVMap and thus, I cannot have chosen the wrong UV Mapping as well. Moving the position of the lamp did not help. In addition, I could not find out where the override all materials option is, so, I could not test out that option. As such, I really appreciate your help and thanks in advance for your help.
My .blend file can be found here.