Formula recalculations when a cell within the formula changes is not working

[donnadindin]

cell 27 formula     =SUM(C3:C24)

any change to cell C3:C24  does not change a previous calculation

What is highlighted? What format am I suppose to use here?

[#2]

@donnadindin 

Make sure that Calculation Options on the Formulas tab of the ribbon is set to Automatic.

[#3]

@Hans Vogelaar  what is the ribbon?

[#4]

@donnadindinokay I found it