Hi,
FlexCel uses english locale for all the formulas. That not only means that it is "ROUND" and not "REDONDEO" or similar, but also that the parameter separator is a comma ",", not a semicolon (the semicolon is used in langauges where the , is used for the decimal separator, so it if you write 2,2 it knows it is the number 2.2 instead of two 2s separated by a parameter).
Try with:
=ROUND(SUM(J2:J125), 2)
Also note that you don't need to use SerCellFromString to enter formulas, it is better and more efficient to enter:
Delphi:
SetCellValue(Row, Col, TFormula.Create('=ROUND(SUM(J2:J125); 2)')
C#:
SetCellValue(Row, Col, new TFormula("=ROUND(SUM(J2:J125); 2)")