Your social security benefits are judgment proof which means they cannot be seized by creditors. The only entities that can touch your SSA benefits are the IRS and Family Court for unpaid child support.
I presume the way this question is asked that this is a valid debt that arose before you turned 65?
The credit card company can sue you -- and as a previous answer states -- your social security benefits might be exempt, but if you have other assets they may be able to levy on those or get a judgement against you and your estate. You should contact a local attorney and explain the situation. Most attorneys will offer a free consultation or a low cost consultation, but your situation may not be as simple as "you are judgement proof."
Since it appears you do not dispute you owe the account, you may still need to hire an attorney to keep the credit card company from getting a default judgment against you. Although your social security benefits are exempt, some of the attorneys that try to collect these credit cards will seize your bank accounts, and if your social security benefits are directly deposited, it may take months to get the money back. I would encourage you to consult with an attorney before your 20 days to respond expire.