A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Usually, it means that one or both of E4 and E5 are text that Excel cannot interpret as a number.
Use =ISTEXT(E4) and =ISTEXT(E5) to confirm.
[EDIT] Better: =ISNUMBER(--E4) and =ISNUMBER(--E5). Note the double negate ("--"). If FALSE, Excel cannot interpret E4 and/or E5 as a number.
You cannot rely on visual inspection or cell format. We can enter text into cells that have a numeric format (anything but Text).
And looks can be deceiving; for example, the text might include non-breaking spaces (ASCII 160; HTML nbsp), which we cannot see.
If you expect text sometimes and you simply want the formula to work sensibly, enter the formula =SUM(E4,E5) instead.