Because the IRS has not yet finalized one of the forms and/or schedules that is included in your tax return for tax year 2020.
Click on Review. A list of forms and the date they will be available are shown,
Go to this website for IRS forms availability - https://care-cdn.prodsupportsite.a.intuit.com/forms-availability/turbotax_fed_online_individual.html
It means exactly what it says, some forms are not ready. There are a lot of last minute tax changes this year. You can look up predicted forms availability for your situations here.
The IRS does not open for e-filing until February 12, so check back in a couple of weeks. There's no rush, and this year there are so many last minute changes that this is not a good year to be rushing.