I filed an extension for 2018, however I received a notice from the IRS. Where can I find a copy of that extension so I can figure out the problem? It is not in my TT doc

[Critter]

Easy Extension Form 4868 doesn't print with my return in TurboTax Online

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.

Solution

1. Log in to your account.
2. Select Add a State to get back into your return.
3. Once in your return through the Add a State button, select Tax Tools in the left menu.
4. Then go to the Print Center.
5. Select Print, save or preview this year’s return.
6. Once you're in your return and you can see Form 4868, select Refresh in your browser.
7. After you've refreshed, print again.

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_historica...

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.