Dear Hamid Reza,
DTTorSpr in FAST is the equivalent torsional spring stiffness of the drivetrain, expressed relative to the low-speed shaft. This can be derived from knowledge of the drivetrain materials (steel), shaft diameters, shaft lengths, etc. The relationship between DTTorSpr and the natural frequency of the drivetrain torsion mode is explained in the following forum post: Drive train natural frequency.
DTTorDmp in FAST is the equivalent torsional damping of the drivetrain, expressed relative to the low-speed shaft. It determines the structural damping of the drivetrain torsion mode.
Best regards,