If you're talking about the error GreenBeret posted about, I think his error is not having added a mapframe yet.
Try following steps 2 to 10 of the tutorial I linked and see if that works. Make sure your terrain doesn't have a georeference if you want to make things easy on yourself.
Mapframe problem with coordinates / UTM Zone
in ARMA 3 - TERRAIN - (BUILDER)
Posted
Thanks for your advice! I fixed it by importing my heightmap in L3DT and exporting it again without georeference.