pbroverride doesn't turn pbr on and off, it allows gameguru to treat non-pbr models as pbr with default additional maps based off the material index.
For terrain just put a neutral flat normal map (all one colour, i forget what the neutral flat normal colour is right now though) named the same as the texture with the suffix _N or _normal, in the same folder is the terrain texture you want to use.
Edit: Attached an example texture to use as a normal map with no "bumps" (it's not really bump but normal directions but you know what i mean) in dds and png, just change the name part of whichever you want to use to the same as your texture.
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