Mpirun was unable to find the specified executable file

GROMACS version: 2020.4
i tried to get replica-exchange by running these commands for 20 temperatures

for i in for i in 0 … 19
gmx grompp -f nvt_$i.mdp -p -c em.gro -o remd$i.tpr;done
mpirun -np 20 gmx_mpi mdrun -v -deffnm remd -multidir sim1 … sim 20 -replex 500

but got

mpirun was unable to find the specified executable file, and therefore
did not launch the job. This error was first reported for process
rank 0; it may have occurred for other processes as well.

NOTE: A common cause for this error is misspelling a mpirun command
line parameter option (remember that mpirun interprets the first
unrecognized command line token as the executable).

Node: quad
Executable: gmx_mpi

20 total processes failed to start

As the error suggests, your gmx_mpi binary is not in the path. Either source GMXRC or set up your path through other means (e.g. modules).