ERROR: Compute used in variable between runs is not current

Dear,
After the model is relaxed, I want to output the time-coordinates of the two groups P1 and P2 in a specified file.
I added the command fix extra1 all print 100 ‘’(c_1) (c_2)’’ file pointP1-P2.txt, and the error occurred.
I have tried to change the order of commands and convert c_xx to v_xx, but that didn’t solve the problem.

ERROR: Compute used in variable between runs is not current (…/variable.cpp:1341)
Last command: fix extra1 all print 100 ‘’(c_1) (c_2)’’ file point16-20.txt

Best.

here is my script:

units metal
boundary f p f
atom_style bond
read_data GL-200X40A.data

pair_style rebo
pair_coeff * * CH.rebo C
mass 1 12.01
timestep 0.0001
neighbor 2.0 bin

I have no problem running the same computes and fix print command with the latest LAMMPS version 3 March 2020.

Axel.