Excel displays the formula text instead of calculating it.
Fix this with AIWhen a cell shows =SUM(A1:A2) literally instead of a result, the cell is formatted as Text, or Show Formulas is on, or the formula is missing its leading =.
Change format to General
Set the cell to General, then re-enter the formula (press F2 then Enter).
Turn off Show Formulas
Press Ctrl+` (grave accent) to toggle formula display off.
Most often the cell is formatted as Text. Switch to General and re-enter the formula.