You have to send them a copy of your check and maybe bank statement showing the payment. Maybe they applied it to the wrong year? Did you send it with the 1040V voucher or was it a 1040ES estimated slip? Seems very late to be getting a notice about a 2019 tax return. Maybe they changed your return and you owe more?
Check your IRS account. Request a transcript from the IRS
You will have to look at your bank statements to see if the check was cashed by the Payee. If you have a copy of the statement showing the payment then contact the IRS using the phone number in the notice you received. Your cancelled check and statement is your proof of payment.
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