Games : Chunk Loaders In Vanilla Minecraft?

I am creating a custom map in Minecraft, but when I try to set a block, I get the error, "Cannot place block outside of the world." I do believe that this is due to chunks not loading. Is there any kind of Vanilla Chunk Loader that I can use to keep many chunks loaded at the same time? I would prefer if there was no clutter.

I have heard about the Hopper system (I don;t know if it stills works, but then again, it creates too much clutter.