There are three different things you can try:
- you can try using
mpiexec -localonly 16 lmp -in in.input -log out.output - you need to properly register the launching services from an administrator shell and then reboot before trying to launch parallel jobs
- you uninstall both packages and instead install the more recent MS-MPI package for MPI from https://www.microsoft.com/en-us/download/details.aspx?id=100593
and a corresponding LAMMPS installer from: LAMMPS Binaries Repository: ./lammps-testing
For more information about the third option (which will soon be the only option), please read this post: LAMMPS Windows Installer Packages with support for MS-MPI - Testers needed