If you are reinstalling, you may need to clear old registry keys to allow a new activation.
Are you looking to to the standalone version, or are you having trouble activating a module you just bought?
If you bought the module on Steam, you must link your Steam account to your DCS account on the official website to use it in the standalone version. 🚀 Ready to Fly?
Ensure you have the base "DCS World" client installed (Stable or Open Beta).