If the property manager did not put locks on the windows and due to that we were robbed, can I hold the owner responsible?

Here your landlord committed gross negligence by not having locks on the windows. However, if you noticed that there were no locks prior to move in, you may also bear the burden or part of the damages because you knowingly left belongings in there when you had knowledge there was no secure lock or there were no secure locks on the windows (not sure if screens would constitute negligence in and of themsevles). However, if you were unaware of the lack of locks, then it would not have been your fault and it was only through the fault of the landlord this loss occurred. Make sure you communicate with your landlord in writing, do not move in or keep anything expensive or irreplacable in there without locks being installed. Make sure you also file a complaint with the agency or agencies in your state or county or city who handle landlord tenant consumer protection matters.