Dwarf fortress construction suspended - builder occupying site

This is a problem with the current version of the game (v0.34.11 as of this writing) and usually happens when the tiles around the place you want to build the wall are walkable, but can't be built from. One such example are ramps.

In your case it's more likely that the path finding gets the Dwarf to build the wall from the "wrong" side (one which triggers the error; usually north). Try those steps:

• Remove all ramps around the building area, even the stand-alone ones sometimes left over from cleaning upper levels by channelling from above.
• Cancel the build order, re-add it a few moments later (pause, cancel, step a few frames, re-add the order, unpause). Since the building material should already be in place (distance: 0), this is fast.
• Set up suspended(!) wall build orders from all sides but a diagonal one to the wall. Dwarves will prefer to stay on the tile which has no build designation, and can build walls diagonally. Remove the suspended orders when done.