can capacitance of a parallel plate capacitor be negative ? if it is not ,why?

Probably your sample is very conducting and you are measuring the inductance of your sample. Negative capacitance is usually a sign for strong inductive contributions to your impedance. Capacitive and inductive contributions to a sample are exclusive. You can only see one or the other, at least at fixed frequency. Can you increase the resistance of your sample somehow? Maybe by cooling? Then you should see the capacitance.