Your object has three materials.
Go into edit mode, then select the materials one after each other and press the select and deselect buttons to find out, which material is assigned to the object. Right now, only the first material mug.002 is assigned to the object.
This material is set up wrong. It you can not plug a color output in the Surface - input of the Material Output node. The Surface input needs a BSDF output, like from the Diffuse BSDF node.
Fortunately, you already have a properly set up material for the object: Material. In edit mode, select all vertices A, then select the material Material and press Assign.
The material will show correctly.