what is the difference between "ligand.prm" and "ligand.itp"?

I guess your parameter file name in your working folder is "ligand.itp". If that case, just remove the #include "ligand.prm".

Khabib Khumaini, Thanks, my parameter filename is topol.top. I have applied your suggestion and it pops up another error.

Fatal error:

Syntax error - File topol.top, line 19

Last line read:

'[ moleculetype ]'

Invalid order for directive moleculetype

I have several questions for clarification:

1. What is your ligand parameter name? is "ligand.itp" or "ligand.prm" or both?Is the files exist in the working directory?

2. Can you provide the last edited topology?

Thanks