Hello! Given that there isn't more info, I can suggest that this error comes either from:
an unexistent terrain texture (ie the terrain is assigned a texture that doesn't exist anymore)
or a pipeline issue that causes some shading/rendering problems: this can happen in particular if you use other Unity versions and had to "upgrade"/"downgrade" the project... take a look at the bottom of the README in this repo for more details
I hope it helps,
else feel free to drop in more data on your issue!
Cheers.