@DoninGA yes, it is to be entered, but if you don't have the spouse's SSN, that does not preclude filing Separate. It does preclude elctronic filing, though, as it is one of the filing edits.
Simply paper file, filing Separate without the SSN.
https://apps.irs.gov/app/vita/content/globalmedia/4491_filing_basics.pdf
this link is from the IRS training material for the VITA / TCE program - look at the TIP box on the top of page 5 - SSN is not required if you are filing Separate, but must paper file. 🙂
