Actually i am using the compute command like
compute 1 all msd/nongauss com yes
and fix command like
fix 1 all ave/time 100 5 500 c_1 file msd.dat mode vector
But when i am running then it is giving error: Invalid compute command.
it works for me using the current version of lammps when i add those
two lines to, e.g. examples/melt/in.melt
are you sure these lines are causing this error?