to see the result of Debug.print use the immediate window:
View, Immediate Window or Ctrl + G

A modification to handle when there are <20 rows:

Sub SumLast20RowsInColA()
Dim lrow As Long, ans As Long, lastEntries As Integer
lrow = Cells(Rows.Count, 1).End(xlUp).Row
lastEntries = 20
If lrow < lastEntries Then lastEntries = lrow - 1
ans = Application.WorksheetFunction.Sum(Cells(lrow - lastEntries +
1, 1).Resize(lastEntries, 1))
Debug.Print ans
End Sub