Ok so as you're using a MixRGB, you can indeed mix the 2 Image Texture nodes into the MixRGB, then you need to plug the MixRGB into a Diffuse, then the Diffuse into an Output:
MixRGB is used to mix 2 colors and/or images, but you need to tell Blender what kind of shader it is supposed to be, between Diffuse (which will be affected by light and shadows) and Emission (which emits light)