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.

https://ttlc.intuit.com/community/forms/help/irs-forms-availability-table-for-turbotax-individual-pe...

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.