Some forms are not yet finalized by the IRS. You
can click on Review on the left-hand side of your TurboTax Online screen and
your forms and dates available will be displayed, here can also sign up for an email notification, there will be a link that will allow you to get email
notifications and updates on your form(s) availability. This way you will
know right away when you can complete your transmission to the IRS.
Here is a list for your reference:
https://ttlc.intuit.com/replies/3319876