The IRS simply wants to make sure it was you that filed your 2018 tax return.
https://www.irs.gov/individuals/understanding-your-4883c-letter
There is a phone number listed in the above link. I would call the phone number and provide the IRS with the information they need to confirm your identity.
Please note if you left the US before December 31, 2018 and used TurboTax, you must include your worldwide income for calendar year 2018. If you earned foreign income after leaving the US, you can use foreign tax credit to help offset any double taxation. If you did not declare your worldwide income for the entire year, you need to amend your return.
