Adding an organic molecule to OPLS-AA force field

GROMACS version: 2023.4/2024.4
GROMACS modification: Yes/No
I want to simulate a polymeric organic molecule using OPLS FF. I tried adding an .itp file to the force field but it shows the following error:

*Back Off! I just backed up topol.top to ./#topol.top.2#

Processing chain 1 (2 atoms, 1 residues)

Problem with chain definition, or missing terminal residues. This chain does not appear to contain a recognized chain molecule. If this is incorrect, you can edit residuetypes.dat to modify the behavior.
8 out of 8 lines of specbond.dat converted successfully

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Program: gmx pdb2gmx, version 2021.4
Source file: src/gromacs/gmxpreprocess/resall.cpp (line 597)

Fatal error:
Residue ‘MEH’ not found in residue topology database

For more information and tips for troubleshooting, please check the GROMACS
website at Common Errors — GROMACS webpage https://www.gromacs.org documentation
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MPI_ABORT was invoked on rank 0 in communicator MPI_COMM_WORLD
with errorcode 1.

All the parameters were generated using LigParGen and rechecked with Gaussian. However do I need to make an .rtp file for the same?

Please let me know if there is another way of introducing a new organic molecule to a forcefield.