If you are using TurboTax Online, when you login you should see a blue button that says "Take me to my return". Click on that link and you will see the reason and steps to follow there to fix your return.
If you are using TurboTax Desktop or Download version, find the TurboTax 2018 icon on your desktop or type TurboTax in your search. Once the program is open, click on file in the upper left hand corner and select Open Tax Return from the drop down menu. When your return is open you should be prompted with the steps to fix it.
Please see the below TurboTax FAQ for additional information on how to fix a rejected return.
https://ttlc.intuit.com/replies/3302275
[Edited 03/26/19 | 5:21 PM]