"Atomtype O not found" error in gromacs 5.1.1

Question

[SS]

Hello,

I am doing MD simulation for Protein-ligand docked complex using Gromacs 5.1.1. When I try to run the following command, I am getting the error " atom type O not found". I used OPLS all atoms force field.

gmx grompp -f ions.mdp -c solv.gro -p topol.top -o ions.tpr

Fatal error:
Atomtype O not found

How to solve this error? I am new to gromacs. Your suggestions would be appreciated! Thank you!