If you are already running a trial version or skipped the key during setup, follow these steps:
Here is a comprehensive, step-by-step guide on how to enter your product key in Visual Studio 2013, along with troubleshooting tips for common activation hurdles. how to enter product key in visual studio 2013
You are trying to use a Professional key on an Ultimate installation, or vice versa. Ensure your key matches the specific edition (Professional, Premium, or Ultimate). If you are already running a trial version