Heightmap Creation Workflow

I am fairly new to Unreal and 3DS Max, so bear with me.

I exported a Revit topography file as an .fbx file.

I then imported it into 3DS Max, with the intent of creating a hieghtmap from it, to import into Unreal Engine. I followed the tutorial below:

When I go to Render-to-Texture, the exported heightmap comes out solid white.

Does it have to do with the size of the mesh not being a square?
If so, I assume there’s a way to modify my import mesh to be square in shape?

See below for the .fbx and 3DS Max files:…lK?usp=sharing

Any help would be appreciated