how do I add normals to terrain shader in 2018.4?

I’m getting this error:
6173596--675751--upload_2020-8-5_16-27-4.png

Unity terrain is peculiar in that it does not have any per vertex tangent data. Instead it calculates the appropriate tangent on the fly in the vertex shader.

If you look at the built in terrain shaders, they all have this in their surface shader pragma line:
vertex:SplatmapVert

That’s this function: