Need advice

I have been working at Google for 11 months now. I had my Q3 performance check when my manager, along with the HR in the room, directly offered me a separation package of 17 weeks saying i was underperforming. It was a shock to me as I did not receive any feedback in the last 2 months from my manager or the stakeholder that I was working with despite of my multiple feedback requests. I want to give myself another chance by improving my performance but my manager denied signing off on the metrics that I created to work on for a month showing all the deliverables, thus keeping everything subjective and in her hands. I feel this is unfair to me and I can sense that they are not ready to give me a fair chance of improvement because the HR denied that this is PIP process and also told me that they do not have any documented PIP policy. How can a company like GOOGLE do like this, which is famous for its culture. I am still unable to process this and in great mental stress. Should I accept separation package or negotiate on that?