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