Create a new sheet within an existing spreadsheet google sheets

Here is my third (and hopefully last) question for my current project.

The IF ELSE statement doesn't seem to be working properly in the following code:

function onFormSubmit() {
  // onFormSubmit
  // get submitted data
  var ss = 
SpreadsheetApp.openById("1UMSTyjYz2DMubBT54Q7a0V5ie5TAPYFZ2riqV5p93gE");
  var sheet = ss.getSheetByName("Submissions");
  var row = sheet.getLastRow();
  var Col = sheet.getLastColumn();
  var headings = sheet.getRange(1,1,1,Col).getValues();
  var lastRow = sheet.getRange(row, 1, 1, Col);
  var UnitNumber = sheet.getRange(row,3,Col).getValue(); 
  var newSheet = sheet.getRange(row,4,Col).getValue();
  // check if username has sheet

  if(ss.getSheetByName(UnitNumber)){
    var DrillSheet = ss.insertSheet(UnitNumber);
    // if not make
  } else {
  var DrillSheet = SpreadsheetApp.create(UnitNumber); 
DrillSheet.getSheetByName('Sheet1').getRange(1,1,1,Col).setValues(headings);
  }
  // Rename sheet to submission date
  DrillSheet.renameActiveSheet(newSheet);
  // copy submitted data to Drill sheet
  DrillSheet.appendRow(lastRow.getValues()[0]);
  DrillSheet.appendRow(['=CONCATENATE(B6," ",B5)']);
  DrillSheet.appendRow(['=TRANSPOSE(B1:2)']);
  //Hide top rows with raw data
  var hiderange = DrillSheet.getRange("A1:A3");
  DrillSheet.hideRow(hiderange);
  //Widen columns
  DrillSheet.setColumnWidth(1,390);
  DrillSheet.setColumnWidth(2,700);
}

The goal here is to create a new spreadsheet with the title 'UnitNumber' and sheet title as the submission date IF that spreadsheet doesn't already exist. If the 'UnitNumber' spreadsheet DOES exist, I would like to create a new sheet within that spreadsheet with the title as the submission date.

Currently it is creating a new spreadsheet every time.