There can be various reasons because of which license key is not getting validated. You can go through one by one to find out if any of the mentioned point is blocking key validation
i. Check if any antivirus is installed in system. If yes, uninstall antivirus as many times antivirus blocks key validation (You can also check by disabling antivirus but mostly antivirus works in background so it may not give 100% result).
ii. Change the internet connection and check.