Hi Ankur,
Ideally invoice simulation doesn't do any 'actual' posting other than creating a simulation invoice document, but I am curious to see why you are getting this error?
To help you further, may I know which transaction are you using to simulate invoice and also the parameters you use? If from EA19, are you sure you have selected the check box below showing 'Document simulated'?
Thanks,
Srini
SAP IS-U Consultant