An extension for a 2018 return gives you until October 15, 2019 to file your 2018 tax return. You can add your W-2 to the return you are still preparing. You might need to click on Add a State in order to get back in to the return. You will not really be adding a state, but that will allow you to go back and add the W-2 to Federal>Wages and Income.
When you requested your extension you were supposed to pay your estimated tax due. Did you enter the payment you made into your tax return? Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes This will show up on line 17 of your Form 1040
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