The values in column D are probably text. Confirm with formulas like =ISTEXT(D2). A visual inspection can be misleading, and the cell format does not matter.
A quick-and-dirty solution is: =SUMPRODUCT(--D2:D5). But other references to column D might cause problems.
It would be better to replace the text values with numeric values. Often, you can use Text To Columns. Select D2:D5, click Data > Text To Columns, then press Finish.
FYI, the Function Wizard (fx) works because it is smarter (albeit misleading) in interpreting the cell values.