Regenerate .tpr file

GROMACS version: 5.1.2
GROMACS modification: Yes/No
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Hi everyone,
I want to calculate free energy g_mmpbs 5.1.x. I had MDS in gromacs 2020.1 then I used gromacs 5.1.2 to re generate index.ndx and md.tpr

I used these commands:
gmx make_ndx -f em.gro -o index1.ndx

1 | 13
q
gmx grompp -f md.mdp -c npt.gro -t npt.cpt -p topol.top -n index1.ndx -o md_0_10_1.tpr

Then I have error:
Attempting to read a checkpoint file of version 21 with code of version 16

Is it necessary to regenerate npt files or I need to use other gromacs version to re generate tpr file? Please help me, thank you so much