This question already has an answer here:
This Pink Material shows that you added an Image-Texture without adding an Image. Go to Node-Editor and check your Material properties (Added Image correctly? Connect all Shaders and Outputs?)
Use the small Button in the corner down on the left side and choose node-editor.
I made an example for how the "Node-Tree" should be although you should choose the "UV" output from "Texture Coordinate" if you've wrapped your Mesh (Not Generated as I did).