Frequently facing Load generator issue "Unable to to play the scenario due to no load generator available" eventhough load generator available.
Usually you got that error message when you start a test with a load generator that is not ready. If you use multiple populations make sure that none of them has a load generator checked which is not with the green light.
You said that it happened frequently so i guess it's not always the case. You may have some network connectivity issue between your controller and your load generator.