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
https://www.irs.gov/individuals/get-transcript
