I have tested this and cannot reproduce this error. As long as you enter the number 1 for the soil model and then proceed to fill out all 8 columns for the user-defined stiffness and ultimate force the conversion to a buried input file should proceed normally. Note this was only tested for versions 4.40 and the upcoming 4.50.
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Loren Brown
Director of Technical Support
CADWorx & Analysis Solutions
Intergraph Process, Power, & Marine
12777 Jones Road, Ste. 480, Houston, TX 77070 USA