In order for excel to show a sum or average in status bar, at least two numerical values should be selected. If not, no value will be displayed as sum or average as in below example.
External links for other issues that might prevent Excel from showing sum in status bar
How to solve if status bar is not displayed
Excel 2007 status bar sum does not indicate positive / negative sum
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Sum not showing up in status bar
Excel not showing sum at bottom bar
Excel not showing total in status bar
Excel 2007 not displaying total in status bar
Total not displayed in bottom of Excel 2010
Summation not possible with Excel status bar
Summing not available in status bar