You may not be in the correct account. That's not uncommon.
When you log back into your account, if there is no "Take me to my return button", and if there's nothing in "Your Tax Returns & Documents" for 2018, and no "Add a State" choice, then you may have multiple accounts and are in the wrong one.
What do you see when you first log in at the Tax Home page. Is there any "Return Status" shown or any evidence you started a 2018 return in that account?
If there is no evidence at all that you have prepared a 2018 return in that account, we can tell you how to troubleshoot for multiple accounts.
A service code is entered at the time of paying for the product when going through the FILE screens.