I am experiencing the same issue. Mine seems to only be on my multyplayer server. Ive moved the markers closer and lower to the ground... but to no avail. I have even gone down and by hand leveled out the bedrock up to lvl 7 in hops that covering up the bedrock might help... No go. The box, stairs, and fill options all work but when I try to use flatten I just get the red bar. I can get it to work in single player. Any help would be grateful.
Thanks