If you are age 65 or older the Form 1040-SR would have been provided for your 2021 tax return. The Form 1040-SR and the Form 1040 are Exactly the same form with the 1040-SR just having a larger typeface on the form.
You can print the form to send to the address requested on the IRS notice you received. Make sure that you sign and date the Form 1040-SR on page 2 of the form before mailing.
To access your current or prior year tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the section Your tax returns & documents. Click on the Year 2021 and Click on Download/print return (PDF)