Atom OXT in residue DG 1 was not found in rtp entry DG5 with 31 atoms while sorting atoms

Hi Pooja,
first of all, if you do not have a strong reason to do otherwise, I’d suggest to use a more recent version of GROMACS. Version 5.1.4 (released in Sep. 2016) is seriously outdated and unmaintained.
I have tried to reproduce the error with GROMACS version 2021.5, using the FF release jul2021, with the fix to the COO- terminal (described here) applied, and ‘THP1 Protein’ added in the residuetypes.dat file. This way pdb2gmx did not complain.
For the sake of curiosity, I’m compiling version 5.1.4 to be able to reproduce the error, but seriously, consider using a more recent release of the code.
Kind regards,
Andras