Fix viscosity is part of the MISC package. Have you installed it? See these two pages (http://lammps.sandia.gov/doc/fix_viscosity.html, http://lammps.sandia.gov/doc/Section_start.html#start-3) for more info.
Fix viscosity is part of the MISC package. Have you installed it? See these two pages (http://lammps.sandia.gov/doc/fix_viscosity.html, http://lammps.sandia.gov/doc/Section_start.html#start-3) for more info.