The fix viscosity command explains what it outputs. If
you read the Muller-Plathe paper you will see it is the
quantity needed to calculate viscosity, along with other
parameters from your simluation.
There are at least 3 ways to compute viscosity using
LAMMPS, as discussed in Section 6.21. You might
try them and compare your results. I know of no
reason that liquid metal, e.g. with EAM, should be different than
other liquids for measuring this value.
Sounds like you are a newbie to the viscosity calculation topic and
also know of no reference whatsoever about viscosity calculations for
liquid metals. My advice:
Take a look at the paper "Journal of Chemical Physics, 2010, 132,
014103" by Tenney and Maginn. As far as I know
Tenney is the author of the RNEMD implementation in lammps thus you
could try retracing his steps in that paper to learn about the RNEMD
method while having a benchmark system to compare to. He used a LJ
particle system (much simpler than your liquid metal) with all
parameters needed reported in the publication. Reproducing some of his
numbers should provide you with enough confidence to trust (or not)
your liquid metal results.