Height Map Help (Not Exporting)

I have been trying to edit the map West Coast and make it my own, one thing that I wanted to implement was some offroad trails and mud pits and bogs. I have studied how you guys do this in GridMap aswell as searched the forums on how to make mud with HeightMaps. HOWEVER I get stuck on the very first step which is (export terrain heightmap) and I dont get a terrain to export
I paint my mud texture and try to export the height map so I can get the mud layer .png BUT I cant export it! (This picture is from GridMap as I was trying to experiment and get any map to export after painting a texture)

PLEASE help me or explain a way to create my mud layer.png a different way so I can get some deeper mud pits.