Unable to use /fill under y=0

I am unable to use /fill under y=0 on a world with cheats enabled. I’m trying to use /fill on a super flat world that is under y=0 for a big project. Whenever I try to ender the y coordinate as a negative, it says, “ Cannot place blocks outside of the world”. The coordinates I am standing on are 600, -60, -99. The full command is /fill 763 -61 -107 600 -61 -99 grass_path.