OpenFOAM can not find (Windows/Microsoft) MPI libraries

I have installed Openfoam with MPI, however everytime I run simulation with Openfoam, the command window would warn that:

"Could not find mpirun or mpiexec, reverting to serial mode. Please install Microsoft MPI to enable parallel simulations.(https://www.microsoft.com/en-us/download/details.aspx?id=100593)."

I just want to run in parallel but it is beyond me why the parallel would fail? Could you kindly offer some instructions to the problem?

The external solvers OpenFOAM, SU2, and FEniCS all depend on MPI (https://en.wikipedia.org/wiki/Message_Passing_Interface) libraries to enable solving problems in parallel.

The interfaces are typically set-up automatically when installing the external solvers. If they are not you must install MPI libraries, which depends on your operating system (OS).

  https://www.featool.com/doc/openfoam.html#install_of_mpi

With OpenFOAM it is recommended to use the ESI distribution https://www.openfoam.com/download/openfoam-mingw-cross-compilation, which should include MPI libraries. Otherwise, as suggested in the error message Microsoft provides Windows compatible MPI libraries via https://www.microsoft.com/en-us/download/details.aspx?id=100593

You can test if the MPI libraries can be found by running the following commands in Matlab

  if( ispc() )
    [st1,res1] = system('mpiexec /?')
    [st2,res2] = system('mpirun /?"')
  else
    [st1,res1] = system('bash -c "mpiexec -V"')
    [st2,res2] = system('bash -c "mpirun -V"')
  end

if either of "st1" or "st2" return zero (0 = no system call error) MPI libraries could be found.