lastRow - 1

function myFunction() {
  var sheet = SpreadsheetApp.getActiveSheet(); 
  var lastRow = sheet.getLastRow();
   
  for(var i = 2; i <= lastRow; i++) {
    if(!sheet.getRange(i, 2).getValue()){ 
      Logger.log(sheet.getRange(i, 1).getValue());
      sheet.getRange(i, 2).setValue(true);
      break;
    }
  }
  
  if(i >= lastRow) {
    Logger.log(sheet.getRange(2, 2, lastRow - 1).getA1Notation());
  }
}