I’m currently flying a charter mission in a Cessna 172. I’ve been told to climb to 20,000 feet which I know I cannot do because I earlier played a mission that asked me to climb to 15,000 feet and I could just about barely get up there. It worked early in the flight because I could just request 10,000 feet but now I was told once again to climb but I can no longer request a lower flight level. I now have to listen to the controller telling me to expedite my climb to an impossible flight level.