Why can I never fill the last block? - getting "unsuitable grade for construction" error
(https://lh5.googleusercontent.com/--vN8kF8IXak/TqNZckBT5aI/AAAAAAAAASk/rspQCBb7Hqg/w563-h359-k/3C%2Bsmooth%2Bcurve.jpg)
This is a very good question. Usually to solve this I have to draw out the diagonal segment out far enough for a 6C starter piece to "take", then destroy the starter (and the straight segment you need to connect it) and redraw the diagonal.
Was the area completely flat before you started construction? If a puzzle piece is built on uneven terrain, it sometimes makes it impossible to build next to parts of it.
I have found with RHW, its easiest to just do the road flattening trick out 3-5 squares from the RHW(or I also use Peg's leveler), this way it avoids the issues you are having, and then when done laying out the highway, I go back and re-level the ground to match what was leveled
What worked was leveling the entire area - which is a must for most NAM/RHW projects.