Problems with generating height maps from mesh

Hi I’m pretty new to blender and 3d programs in general, so if you post help could you make it simple and clear.

Basically i have a terrain mesh, and its non-manifold. So rather than spend hours trying to fix it i thought it would be a great idea to create a height map and use that to recreate the mesh. The problem is that after following this tutorial (link not provided due to posting restrictions) when i click to render i only get a solid grey image.
I tried the tutorial example file and had no problems with it.

So would this be an issue with the mesh being non-manifold ?
Any help would be great, THX

This tutorial.