I used the input file in the examples/reax directory called in.reaxc.rdx and it gave error. The other input (in.reax.rdx) worked.
In /src type “make package-status”. If it says “Installed NO: package USER-REAXC” then type “make yes-user-reaxc” and recompile. If it says “Installed YES: package USER-REAXC”, then type “make clean-all” and recompile. If that doesn’t work then checkout a new tarball from lammps.sandia.gov and try again since something is screwed up.