Dear Mohamed,
The error seems fairly clear… Your box is being deformed by an amount greater than x length of your simulation.
The cause of this appears to be because you have your times wrong. For equilibration, you say you are running for 10 ps, but 0.001 ps * 20000 steps = 20 ps. Then for deformations, you say you are running for 20 ps, but 0.002 picoseconds * 100000 steps = 200 ps.
Putting this in the deformation equation for engineering strain (L(t) = L0 (1 + erate*dt) from the LAMMPS documentation) with your equilibrated x length gives:
54.488113 * (1 + -0.01 * 200) = -54.4881
Which is below 0—what the error told you.
Michael