You simply wish for far too much! You think that your cubic depends on two (independent) parameters, but you two parameters a and b are NOT independent and in reality your cubic only depends on one parameter and therefore the solution block cannot find a solution.
You cannot demand the starting and end points, as well as the curvature in both points (zero and r0) and use a simple cubic as transition curve. You could, for example, use the x-coordinate of the starting point on the straight line as the second parameter in the solution blocks.
Maybe you have more luck using a clothoide instead of a cubic as transition curve.
