Dear @KanwarOsama.Zulfiqar,
Are you trying to follow the process outlined in the following forum topic for computing the 6x6 rigid-body mass matrix from ElastoDyn: OpenFast 2nd order Linearization?
I see that in ElastoDyn you’ve only enabled 6 platform degrees of freedom, which is correct. But I don’t see that you’ve included the platform load inputs as an input in the linearized system, which requires that LinInputs = 2 in the OpenFAST primary (.fst) input file. While the forum topic I linked to above mentioned how to derive the mass matrix from the input-transmission (D) matrix, this requires that you include the 6 platform accelerations (3 translations, 3 rotations) in the OutList of ElastoDyn, which you have not included. However, the platform accelerations are also included in the first time-derivative of the continuous states (xdot), and so, you can also derive the mass matrix by inverting the 6x6 submatrix of the input matrix (B) associated with the platform load inputs and the platform accelerations, once LinInputs = 2.
Best regards,