Fatal errors in .rtp file

GROMACS version: 2020.1-Ubuntu-2020.1-1
GROMACS modification: Yes/No
Here post your question:

Fatal error:
Atom CG in residue VAL 3 was not found in rtp entry VAL with 16 atoms
while sorting atoms.

I guess you got this error running gmx pdb2gmx.
You can check that the atom CG is defined in the rtp file under the directive [ VAL ].

The error means the residue expects an atom named CG and you don’t have one in the coordinate file. That means your coordinates are incomplete and you need to model in anything that’s missing (outside of GROMACS).