The Micro Exporter only outputs geometre. If that's now what you need, it's not the approach to use.
For height maps/heightfields, you need to use a Heightfield Generate node. If your terrain is Procedural, then you feed the last shader in your procedural terrain network into the Shader input of your Heightfield Generate. Then you set the size of the heightfield in meters and pixels (area coverage and detail, respectively) and Generate. Then right-click the node in the node network and choose Save As. You can save as EXR or TER (Terragen terrain format).