What to do if I was called into the HR office and told that my position no longer exists?

If you want to qualify for unemployment without any issues, then do not sign this document.  A plain reading of it does represent that you are voluntarily leaving-- but you are not.  If you sign an untrue document now, you will have a very hard time undoing it later.  You have a couple of options depending on what you are up for:

First, have an employment law attorney review the entire document to make sure that it does not contain other valuable waivers of your rights.

Or, second, tell them you will sign it, but only if they delete or cross through the "voluntarily" word so that the document will correctly reflect the basis of your departure from the company.  Make sure you keep a signed copy so as to have your proof later that you did not sign a document saying your left voluntarily.

Or, third, refuse to sign the document in its entirety, and go file an unemployment claim instead.  Before you take this route-- go to your state's website and make get an estimate of what you would potentially receive through unemployment.  If the severance package is extremely good, then you may want to choose the severance over unemployment.  Most are not, but a few are...so you should at least give this some thought.   The reason they are offering you a severance is to keep their unemployment expenses down.   The more claims they have to pay, the hire their unemployment insurance premiums will go.  They haven't worried about their job... so don't worry about their premiums unless they are going to compensate your appropriately.