Error in mdrun em

GROMACS version: 2020.04
GROMACS modification: Yes/No
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i am trying to do MDS for the protein only.
When i tried at 2021 version there is no error exist but i’m downgrade GROMACS version for GPU utilization and stability. so this time i run the command with gmx mdrun -v deffnm em or gmx mdrun -v deffnm -nb gpu and I am getting error like this

:-) GROMACS - gmx mdrun, 2020.4 (-:

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GROMACS: gmx mdrun, version 2020.4
Executable: /usr/local/gromacs/bin/gmx
Data prefix: /usr/local/gromacs
Working dir: /home/user/Desktop/kyoungpyo/gromacs_data
Command line:
gmx mdrun -v -deffnm em -nb gpu

Reading file em.tpr, VERSION 2020.4 (single precision)

Program: gmx mdrun, version 2020.4
Source file: src/gromacs/domdec/domdec_setup.cpp (line 789)
MPI rank: 6 (out of 14)

Fatal error:
The number of ranks selected for particle-particle work (14) contains a large
prime factor 7. In most cases this will lead to bad performance. Choose a
number with smaller prime factors or set the decomposition (option -dd)

For more information and tips for troubleshooting, please check the GROMACS
website at Errors - Gromacs

Please help me to fix the error.
Thanks in advance.