Google-apps-script IF ELSE statement

I am beginner in script programming and I would like to write a script in order to have an automatic ID (date+Uppercase characters).

The script for incrementation of character to next character is OK without the function IF. However, the script does not work correctly when I write the IF statement.

Basically: -script without IF statement: result OK ("A" -> "B" -> "C" ... -> "Z" -> "A"...) -script with IF statement: result NOK (only "A")

function onFormSubmit(e) {

//Déclaration des variables
var SheetResponse = SpreadsheetApp.getActiveSheet();
var DerniereLigne =  SpreadsheetApp.getActiveSheet().getLastRow();
var DateToday = Utilities.formatDate(new Date(), Session.getScriptTimeZone(), 'ddMMYY');

//Intégration du suffixe alphabétqiue pour l'ID
// If cells value (A.n)=(A.n-1) then character of cells "N.n" is incremented until "Z" (with n is number of LastRow)
if (SheetResponse.getRange(DerniereLigne,2).getValue() == SheetResponse.getRange(DerniereLigne-1,2).getValue()) {
        var AlphaNumber = SheetResponse.getRange(DerniereLigne-1,15).getValue().charCodeAt(0);
        var NextCode = AlphaNumber + 1;
        // Si Code (Z) alors restart to "A"
        if (NextCode > 90) {nextCode = 65;}
        var NextAlpha = String.fromCharCode( NextCode );
        }
// If not cells (N.n) is set to "A"
else {NextAlpha = "A";}

//Création de l'ID dans la derniére ligne et colonne "N"
SheetResponse.getRange(DerniereLigne,14).setValue(DateToday + NextAlpha);
SheetResponse.getRange(DerniereLigne,15).setValue(NextAlpha);

}

Please, Could someone help me.

Thank you in advance.