You will have to print and mail the tax return if you cannot enter their SS number. Using TurboTax for their SS number enter 999-88-9999 so that you can complete the tax return. Print the tax return for mailing and then erase or white-out their SS number and enter Unknown.
See this TurboTax support FAQ for printing a tax return for mailing - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
