If filing married but separate why is the spouses ssn needed?

[AnnetteB6]

The short answer is because the IRS requires that you include your spouse's name and Social Security number on a Married Filing Separate return.  

If you do not have that information and then you cannot file your return electronically, but it can still be filed by mail.  You should also include a cover letter explaining why the information is missing from your return.  As you go through the process in TurboTax, you may see some error messages related to the missing information.  You can ignore those and proceed with filing your return by mail.