Please always reply to the list.
If you are stuck using an old version of lammps I cannot help you, but maybe someone else can.

With reax the arguments are numbers (the order of the elements in the force field file) and with reax/c the arguments are element types.

If in your force field file “” C is listed as the second element in the list, then you just need to use “pair_coeff * * ffield.c-ireax.CH 2 2”