Megjia,
Looks to me like your input has a problem before you even get to the fix GCMC line. Try commenting out your fix GCMC line and make sure that your input script works without it first, then try turning on fix GCMC.
Here’s a simple example input script (see below).
Paul
units real
atom_style atomic
boundary p p p
lattice fcc 0.1
region box block 0 1000 0 1000 0 1000
create_box 1 box
mass 1 83.80
pair_style lj/cut 10.0
pair_coeff 1 1 0.337824 3.69 10.0
thermo 100
compute_modify thermo_temp dynamic yes
variable chempot equal -2000
variable density equal atoms/vol
variable specdensity equal v_density1.39156E-251e27
variable chempotkcal equal ${chempot}/503.219
fix 2 all gcmc 1 100 100 1 29494 300.0 ${chempotkcal} 0.5
thermo_style custom step atoms f_2[1] f_2[2] f_2[3] f_2[4] f_2[5] f_2[6] temp pe ke etotal press vol v_chempot v_specdensity
run 100000