There can be a number of reasons why the quad will not take off in flight mode and they are all usually related to how the game is installed.
Typically the problem is caused by the sim database being inaccessible for some reason and as a result it cannot read all the parameters it needs to simulate flight.
Usually this problem happens on windows when the main application folder has been copied into the "program files" directory. This is a protected directory and the sim will not be able to read from it's database. The fix is to move the main folder to a read / write location like documents folder or desktop or any normal folder that is not a system folder.
OSX Yosemite has a compatibility issue with the sim and is missing some audio libraries that are required for the simulator to function correctly. The only fix we have found to this is to upgrade the OSX version.
Gatekeeper on some OSX systems will quarantine the sim if it is being run from the downloads folder - this stops database access and causes the problem. Move the sim folder to documents or desktop to resolve.
Not having gatekeeper security set to "all applications" (including ones downloaded from the internet) will cause a quarantine and the problem to occur. Update gatekeeper settings to fix.
Further information on security settings and how to fix here --> https://batcavegames.freshdesk.com/solution/articles/16000084754-gatekeeper-putting-the-sim-into-quarantine
If you are running on a non-english language linux system then this problem can occur, the fix is linked here -> https://batcavegames.freshdesk.com/solution/articles/16000062424-running-velocidrone-on-a-non-english-language-linux-system