The tournament is full. Try registering for another tournament.
You didn’t check in. If you registered for a tournament, you may need to confirm that you’re ready to play. If you missed check-in, you will be removed from the tournament.
The game isn’t installed, or you don’t own it. You can install games from Home > My games & apps. Also, make sure that the game has the latest updates installed (launching the game will ensure that you have the latest version). If the title isn’t in your Games or Ready to install list, you’ll need to get it from the Microsoft Store.
A temporary connection problem may be preventing you from using Arena on Xbox Live. Wait a bit and try again.
You don’t have Xbox Live Gold. To play games that support Arena on Xbox Live, either you or someone on the Xbox console you’re using must have a Gold membership. (You don’t need a Gold membership to register for tournaments.)