I'm facing the same problem. I remove the line in the .itp files (in your case line 55) and i'm able to continue the simulations. But i'm not sure if this is the right thing to do. Better checking the ligand topology.
This error occurs because your used force field and your downloaded force fields are different. Please have a look at your topology file i.e., topol.top . Be very careful while choosing force field using pdb2gmx command.