QUOTE
You can only use the Create action to create units that can be preplaced in normal StarEdit.
OBVIOUSLY!
QUOTE
Items under the "powerups" category can be placed on any terrain that a unit can walk on, basically. If there are units there, most often, it simply creates there anyways and forces the unit to move.
Concerning resources such as mineral chunks and vespene orbs/sacs/cubes, they can only be pre-placed. However, they can be moved, so many maps place a cache of these in a corner of the map to use later.
PCFredZ sums it up, pretty much
Walkable terrains eh?
Well i tried creating it on high terrain which is "walkable"
It doesnt say "Unable to create on 123, 283"
It just dont say or create it at all