May I file without the Payer's SSN (I do not have the SSN) on a seller-financed mortgage for 2nd home I sold?

[Leonard]

You can file without the Payer's SSN and when the IRS notices that you have not provided the SSN they will contact you asking for clarification.  You will need to have detailed records documenting your request for the Payer's SSA.  After providing the IRS with this information they will take action to correct the issue.