You will have to file an amended return to add the 1099 that you omitted. Go to the following link for instructions to amend a 2018 tax return that you filed with TurboTax Online.
https://ttlc.intuit.com/questions/4667226-how-do-i-amend-my-2018-turbotax-online-return
You didn't say what kind of 1099 you have. There are 20 different kinds. There is no plain Form 1099. All 1099 forms have one or more letters after the 1099. The most common are 1099-INT, 1099-DIV, 1099-B, 1099-MISC, 1099-R, and 1099-G. Depending on the type of 1099 that you have, you might not be able to report it in Free Edition. You might have to upgrade to one of the paid editions.
An amended return has to be filed on a special form, Form 1040X. It cannot be e-filed. You have to print it, sign it, and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
Don't file the amended return until your first return has been accepted and you have received the original refund.
