How to diagnose "429 Error" in Google Script - google-apps-script

Question

My script is looping through a list, at each index it is saving a sheet in my workbook as a PDF, then saving it to my google drive folder. There are 7 people in the list. The problem is that my script throws a 429 Error after the 6th PDF is exported. I am trying to figure out how to solve this error but I am quite new to google scripting so I need some help in the right direction.
Error Image
function ToPortrait() {
var ss = SpreadsheetApp.getActiveSpreadsheet();
var validation_sheet = ss.getSheetByName('ValidDSM');
var lastRow = validation_sheet.getLastRow();
var inputs = ss.getSheetByName('ValidDSM').getRange('A2:A'+lastRow).getValues();
var sheet2 = ss.getSheetByName('DSM');
var tab = ss.getSheetByName('DSMD');
var url = ss.getUrl().replace(/edit$/,'');
var sheets = ss.getSheets();
var optSheetId = 1123194386;
var options = {
headers: {
'Authorization': 'Bearer ' + ScriptApp.getOAuthToken()
}
}
// Loop through List of 7
for (var i = 0; i < inputs.length; i++){
sheet2.getRange('B3').setValue(inputs[i][0]);
SpreadsheetApp.flush();
Utilities.sleep(500);
// Loop through all sheets, generating PDF files.
//for (var i=0; i<sheets.length; i++) {
//var sheet = sheets[i];
// If provided a optSheetId, only save it.
// if (optSheetId && optSheetId !== sheet.getSheetId()) continue;
//additional parameters for exporting the sheet as a pdf
var url_ext = 'export?exportFormat=pdf&format=pdf' //export as pdf
+ '&gid=' + tab.getSheetId() //the sheet's Id
// following parameters are optional...
+ '&size=letter' // paper size
+ '&portrait=false' // orientation, false for landscape
+ '&fitw=true' // fit to width, false for actual size
+ '&sheetnames=false&printtitle=false&pagenumbers=false' //hide optional headers and footers
+ '&gridlines=false' // hide gridlines
+ '&fzr=false'; // do not repeat row headers (frozen rows) on each page
// Get folder containing spreadsheet, for later export
var name = sheet2.getRange('B3').getValue();
var folderID = "1IsffOYYQOHZJKggUzRTntfjmJKN0F6ga"; // Folder id to save in a folder.
var folder = DriveApp.getFolderById(folderID);
var response = UrlFetchApp.fetch(url + url_ext, options)
var blob = response.getBlob().setName(name);
//save the pdf to drive
folder.createFile(blob);
}
}
I expect all 7 PDFs to be exported but only 6 are.