Hi, I finished my return for 2018 but never mailed it out due to missing one of my children social security. Am I able to file an extension this late in the year?

[xmasbaby0]

No, you cannot get an extension--the last day to request an extension for 2018 was April 15, 2019.  Now your return is just late.  If you are getting a refund there is no penalty for filing late.  If you owe tax due then you will also be subject to penalties and interest, for which will be billed later by the IRS, after they receive your return and your payment as shown on the Form 1040.

Now the return must be filed by printing, signing and mailing it.  It cannot be e-filed.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

[VolvoGirl]

If you need to add your dependent and their ssn you will need to finish your return.  You need to enter their ssn in the program to qualify for credits like child related credits and the EIC earned income credit.

How to finish a prior year online return

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-...

[Critter]

If this was a newly issued SS# then you can claim the child as a dependent however even though the program will give you the child related credits expect the IRS to disallow them due to the new tax law changes ... the program should ask you when the SS# was issued but it doesn't ... maybe something they will add in the future. 

The Protecting Americans from Tax Hikes or PATH Act made several changes to the tax law, including preventing retroactive claims of the EITC and CTC by amending a return or filing an original return for any earlier year in which the individual or anyone listed on the return did not have a SSN valid for employment. You can't claim EITC or CTC unless the Social Security number for you, your spouse (if married filing a joint return) or a qualifying child is issued before the due date of the return including any valid extensions.

http://www.thetaxadviser.com/newsletters/2016/jan/congress-makes-changes-to-popular-tax-credits.html