If you've seen this message, it means that the League of Legends webpage you're viewing is on a different region than the account you're currently logged in with. You might experience limited functionality on this webpage, like not being able to comment on a boards post or not having access to some match history pages.
This can be resolved by logging into an account that's on the same region as the webpage you're visiting.
You can log into an account on a different region by clicking the drop down next to your summoner name at the top right and selecting Logout. Click Login, and choose the appropriate region from the Region drop down menu. You must have an account on the selected region in order to successfully log in.
You can read more about our separate regions here.