Line 21 is now Sch 1, Line 8.  The input is under Income > SS Benefits, Alimony, Misc. Income.  Scroll down to Alimony and Other Income for Other income (Click on button to expand) and enter the amount under the taxpayer's column.

If it is subject to SE-tax, on the other hand, it should be entered under Income > Business Income (Sch C).