function freezeValuesExecute_(sheet) {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName(sheet);
  var range = sheet.getRange("A:Z");
  range.copyTo(range, {contentsOnly: true});
}

function freezeValues() {
  var date = new Date();
  date.setMonth(date.getMonth()-1);
  var year = Utilities.formatDate(date, "Atlantic/Canary", "yy");
  var month = Utilities.formatDate(date, "Atlantic/Canary", "MMM");
  var sheet = month + " " + year;
  console.log(sheet)
  freezeValuesExecute_(sheet);
}

function doItonce() {
  var date = new Date();
  var month;
  var sheet;

  for (let i = 0; i <= 11; i++){
    date.setMonth(i);
    month = Utilities.formatDate(date, "Atlantic/Canary", "MMM");
    sheet = month + " 24"
    freezeValuesExecute_(sheet);
  }
}