Non-numeric pressure - simulation unstable

can anyone help me with this error “Non-numeric pressure - simulation unstable (src/fix_nh.cpp:1049)”

------------------------- SETTINGS ---------------------------------

compute csym all centro/atom fcc
compute peratom all pe/atom

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EQUILIBRATION

reset_timestep 0
timestep 0.001
velocity all create 300 12345 mom yes rot no
fix 1 all npt temp 300 300 1 iso 0 0 1 drag 1

Set thermo output

thermo 1000
thermo_style custom step lx ly lz press pxx pyy pzz pe temp

Run for at least 10 picosecond (assuming 1 fs timestep)

run 20000
unfix 1

Store final cell length for strain calculations

variable tmp equal “lx”
variable L0 equal {tmp} print "Initial Length, L0: {L0}"

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