Try reading the Schedule A Instructions.  Diagnostics are generally written by programmers who take their returns to H&R Block.  

Seller financed mortgage. If you paid home mortgage interest to the person from whom you bought the home and that person didn’t provide you a Form 1098, write that person's name, identifying number, and address on the dotted lines next to line 8b. If the recipient of your home mortgage payment(s) is an individual, the identifying number is his or her social security number (SSN). Otherwise, it is the employer identification number (EIN). You must also let the recipient know your SSN.