I'm attempting to use OPLS-AA to generate force field parameters for lithium and bis(fluoro sulfonyl) imide (FSI), but when I upload each PDB file separately, I get an error message similar to this:

Sorry, an error has been detected in your input data (file, smile or selected charge):

•           Found residue ligand NIL
• Unknown error. Please, check the input file. If you are not able to find the error, we suggests to use the SMILE code

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• If you can NOT fix it. Please, report the problem clicking in the following link: How can I rectify this error?TIA