DF2014:Dwarf cancels Construct Building: cannot reach site
This error message will show if a dwarf is unable to find a valid path to the construction site. You may want to check if the site is blocked in some way such as on an unreachable z-level, across a body of water, or blocked in by walls or structures.
The other side of this could be that the dwarf that generated the message is stuck themselves. Make sure they have not walled themselves in or have gotten stuck in some other way.
This error may also occur if a dwarf refuses to move an obstructing object in a diagonal corridor. You can manually order the object moved, then finish the construction. This behavior is presumably a bug.Bug:8879