If you cannot optimize your route, there are a few things you can try:
Update the Straightaway app to the latest version. After updating the app on iOS, you might need to restore your purchase.
Check if all stops are in the same area. You might have added a stop on another continent or in another country. Open the Map and zoom out to check if all stops are in the right area. Delete stops that are not in your area and reoptimize your route.
Check your network connectivity. You will need a stable internet connection in order to optimize your route.