Why does my /fill command output "That position is not loaded"?

Question

I am creating a type of adventure map. Once you enter the map, you step on some pressure plates that are supposed to seamlessly close the wall behind you. For some reason this command does not work when in a command block or just in text. Here is the command:

/fill -17 4 -109 -18 5 109 minecraft:light_gray_concrete

The output it gives me is "That position is not loaded". I am on a singleplayer superflat world in 1.14.3.

Answer 1

The error message is pretty self explanatory, it means that the coordinates where you want to place blocks are not loaded.
You probably get that problem, because you used 109 and -109 as z coordinates, and only one of those is close enough to be loaded.

Try to use one of these instead:

/fill -17 4 109 -18 5 109 minecraft:light_gray_concrete
/fill -17 4 -109 -18 5 -109 minecraft:light_gray_concrete