Turbo Tax says it needs to get some other "forms" to complete my return? Why is that necessary?

It is early in filing season, and all Federal and State forms are not yet finalized and available.  Many will become available each week.  You can monitor the status in the availability tables below.

If you are using the desktop version (CD/download), you may need to perform a "check for updates" to be sure you get the latest forms.    The online version should update automatically when form updates are available.

You may be able to tell what forms you have that are not available by opening your return and stepping through the REVIEW tab.

Federal forms availability dates:
https://ttlc.intuit.com/questions/1908854-forms-availability-table-for-turbotax-federal-products-exc...

State forms availability dates:
https://ttlc.intuit.com/questions/1908857-state-forms-availability-table-for-turbotax-individual-per...

The IRS will not start processing returns until January 23 (except for a few test returns the IRS will pull early.).  Any returns filed early will just sit on TurboTax servers until then and cannot be retrieved if you notice something wrong.  You'd later have to file an amended return.  

You must wait on any W-2's, 1099's, etc. to arrive first before filing.

And starting this year, if you have EIC (earned income credit) or ACTC (additional child tax credit), the IRS will not start releasing those refunds until February 15.

Some forms (e.g. SS-8) are just not supported by Turbotax, but TT will tell you that in the interview.