Like @Karl I don't do 1040NR returns. Here is what I did to make it work. 

Screen 14.1 enter the withholding only and I assume there was none. 

Screen 58.1 > Tax on Inc. not Effectively .... > Social Security > Other Percent 1 Column > 85% of the Social Security received (Use a calculator) Page 43 of the Instructions 

@Phillyjkr What form was the Social Security reported on? Are you sure you should be filing 1040NR?