I am guessing that it is Gromacs. Have you checked the force field file file location? Also, when do you get this error exactly? It would be better to put some inputs.
If you want to change your FF, it is better to copy the FF file to your work file directory. As mentioned, there is no amber99sb-ildn-HHH.ff in the FF file. I think this file is your modifed file that is not placed in the true directory.
As Hüsamettin noted, you need to check the file location and the dependencies of this .itp file. If other parameter files are dependent on the files that you made changes to, you will also need to make sure that they "included" the right files.