Dear Boran,
From my quick review of your FAST v7 input file, I see that you have lines pertaining to inputs PtfmAddFile, Kx, …, Br_quad. But these are not standard input parameters in FAST v7. Have you customized the source code in some way to include these? (The addition of these input parameters is likely why FAST v7 cannot properly read the input file and is triggering an error regarding TwrNodes.)
While FAST v7 can be used to linearize floating offshore wind models, the hydrodynamic capability of FAST v7 is greatly simplified relative to FAST v8 (and FAST v7 cannot really be used to model the strip-theory members of the OC4-DeepCwind semisubmersible).
We at NREL are currently working on the extension of OpenFAST to enable full-system linaerization for floating offshore wind turbines, including the linearization of HydroDyn and MAP++.
Best regards,