I filed an extension through you for my 2018 tax return and now the irs says I didn't do that and I owe money.

If you used the TurboTax Easy Extension software, you should have received an email confirmation from TurboTax within 48 hours. Check you email.

Please remember, an extension doesn't give you extra time to pay your taxes. Your taxes are due on or by April 15, 2019. The extension keeps you from getting a late filing penalty as long as you file your tax return(s) on or by October 15, 2019.