Trouble lighting a mesh

Hi all,

So I am trying to make a road scene, where the "road" is a mesh with a parallax scrolling effect. So it repeats itself, scrolling vertically.

I want to put street lights into the scene, however I am having trouble getting the overall directional lighting to work (so I can make the scene dark!). I have made a new material with "Sprites/Diffuse" (as all the tutorials say) but alas the whole Mesh goes white, instead of showing the road image it had before.

I have tried a bunch of different approaches, mostly related to different shader combinations, but nothing has worked to create a lighting effect.

Here is a shanpshot of the main window: https://ibb.co/NsSMp4f

Lighting settings are A black ambient colour and skybox source.

This is my first foray into lighting and so please let me know if there is more I need to give to help diagnose :)

Thanks! Alex