If you're using TurboTax Online, you may experience an issue when printing your 2018 tax return if you also used Easy Extension. When you print, Form 4868 is doesn't populate.
If you did not file an extension thru TT or if it is not there then get an account transcript from the IRS to see if they ever got one from you. https://www.irs.gov/site-index-search?search=transcript&field_pup_historical_1=1&field_pup_historical=1
And is the notice for a failure to file or failure to pay penalty ? An extension cannot save you from the second type ... it only gives you more time to file not to pay.