Can't Place a Layer of Blocks All on the Y-Axis Without Struggling with the Fill Command

I am playing on 1.10.2 in my realm, and I need to place grass along a specific axis so my friends and I can build basements without seeing the bottom of the world (we're on a super flat world) I tried the fill command and it says there are too many blocks, or it says I can't place blocks outside of the world. How do I do this?

Solution 1:

You can only fill small chunks of blocks at a time. You also cannot place blocks, even with fill, lower than bedrock. The only solution would be to create a tall, overground chunk of dirt. You could disguise this as a hill, but it would still stand out among the superflat landscape.