Dear Steve and Aidan,
Thank you for your reply.
I have examined the problem these days. The reasons might be like this:
LAMMPS provided the REAX forcefield of 6 elements: C, H, O, N, S, Si. However, the ffield.reax doesn’t describe all the interactions between any two of the 6 elements, for example, the Si-O bond and O-Si-O angle. So, when I want to do a simulation about Polydimethylsiloxane[ (C2H6OSi)n ], that error occured.