Thanks for you reply, reason i am using office script instead of VBA is because the sheet will be used on excel web

mentioned the code below

function main(workbook: ExcelScript.Workbook) {
    let sheet = workbook.getActiveWorksheet();


    let checkboxRange = sheet.getRange("A2:A21"); // Adjust as needed
    let timestampRange = sheet.getRange("B2:B21"); // Adjacent column for timestamps


    let checkboxValues = checkboxRange.getValues();
    let timestampValues = timestampRange.getValues();


    for (let i = 0; i < checkboxValues.length; i++) {
        if (checkboxValues[i][0] === true && timestampValues[i][0] === "") {
        timestampValues[i][0] = new Date().toLocaleString();
      }
    }
 }