The error you receive depends mainly from the atom type definitions of the forcefield.

You should check wich ff you are using, wich water model you want to use. Moreover the atom types in your pdb condifuration should, for semplicity, be equal to the model you want. I seuggest you to check the forcefield.itp of your gromacs version.

I am using gromos53a6 forcefiled and it contains OW atomtype defined in it