Forwarding your call of an angry customer to your supervisor, does it mean you are lacking in something like, negotiation skills,handling pressure?

Not at all.
The customer is usually a lot calmer when the call is transferred to a supervisor or 'supervisor'.
They have already then been yelling out the worst anger.

It depends, if the customer has been calling more than10 times for the same problem, that is not your fault. It is a fault in the system and it is understandable because the customer has to undergo the same process all over again with another rep so they just ask for a supervisor on the onset of the call. That's why you need to review the cases once you pull up their account. Still try to pacify them, if you were able to, then that is an achievement, but if no, then escalate according to your protocols. Passing it to your supervisor doesnt necessarily mean that you lack something because all calls are not the same, we treat all calls as unique and individualized because no two people are the same in this world when it comes to their thoughts and emotions.

A customer usually asks to be transferred to a superior if they feel that the person on the phone has not been able to address their query satisfactorily.
That doesn't necessarily mean that the agent is inadequate in some way, particularly if he or she has done their best and exercised the best possible levels of customer service.
Most angry customers calm down provided the person attending to the call stays calm, empathises, is genuinely understanding and tries to provide alternative solutions or options and sticks to their promises.
The agent should not take such transferred calls to heart, and understand that such things are part and parcel of the customer services industry where customers are always trying to get their money's worth.
Each and every such call is an opportunity for improvement and getting better at what they do.