Can we sue landlord for rent of a property is not registered and delinquent for two years and failed to notify us?

You can sue anyone the question is whether you can win. The City of Los Angeles Rent Stablization Ordinance allows landlords who get in compliance with the RSO to retroactively validate the rent collected. Thus, if it is a minor technical non-compliance, you will probably lose. If the landlord cannot bring the unit into compliance, then you have a much better chance of winning.