The reduced cost (dual value) is not available in a non-linear optimization problem. Use LpReducedCost on linear programs.
You've passed the function LpReducedCost an optimization problem that was defined with something other than LpDefine -- perhaps QpDefine or NlpDefine. The function LpReducedCost can only be applied to linear programs (LPs).
This error can also occur when the problem is an LP, but the LP cannot be solved. For example, if the LP has an unbounded objective function or no feasible solution. To check for this, examine the result of LpStatusText.