Problems with generating height maps from mesh

Hi I’m pretty new to blender and modeling in general, iv got on with the program quite well but iv just bumped into my first major problem.

I have got a non-manifold mesh, it seemed like it would be pretty complex to correct, so my idea was to generate a height map from this mesh and then use it to create a new mesh without the non-manifold problems.
But whenever i try and render the height map it is just a grey square.

I followed this tutorial
The example file works with no problems.

Any help would be great THX!