Hi Saba,
It seems as though you are using an older version of MRtrix3; version 3.0_RC2 at the latest. The linear histogram matching modes were not yet implemented, and so the “nonlinear” mode specification was not necessary since it was the only mode available. See the difference in help pages between 3.0_RC2 and 3.0_RC3.
Since these version mismatches do come up from time to time, I threw together a Wiki page where we can hopefully more efficiently direct people when these issues arise.
Cheers
Rob