When I use the .set "topBlock" for a biome I receive an error in my log file telling me that BiomeTweaker has tried to locate the block minecraft:grass_block

This block does not exist in MC 1.12.2. It should be just minercraft:grass in that version. It wasn't named grass_block until a later minecraft version.

(I'm also unclear on why it just tries to locate grass, as that is not the topblock in all biomes)

I can't find my old scripts and logs. It was part of a large modded pack which I had to move on with. In vanilla it seems to work fine. I'll let you know if/when I find it again, but the error log definitely stated that BiomeTweaker couldn't find the resource minecraft:grass_block

Apologies - I'll provide more info next time.

I'll need to see the full error and your script.

All right, well I'm closing this issue until you can find those scripts then.

The resource location is "minecraft:grass"!