you cannot unbend solid geometry. Unbend command can only be applied to sheetmetal geometry. Also Unbend command can only be applied to ruled surfaces (zero Gaussian curvature).
please send your part again. I suggest you to build the part in Sheetmetal mode. Do not convert solid geometry into sheetmetal geometry. Also do not forget that Unbend command can only be applied to ruled surfaces (surfaces with zero Gaussian curvature). Please check Gaussian curvature of your model.
