I would go to the IRS website which speaks to the cp63 notice (link provided). Call the number and have them send you a copy. It is not clear from your message if you are speaking about the 2020 return or a prior year return. However, as the 2020 return is not due yet, I assume it is for 2019. I would get your certified mail receipt or any other proof handy. If you had a tax liability and you paid tax for instance, get your bank statement or cancelled check and be ready to point to the debit. Or if you got a refund, point to that too. good luck.
https://www.irs.gov/individuals/understanding-your-cp63-notice
